diff --git a/src/ast/ast_smt2_pp.cpp b/src/ast/ast_smt2_pp.cpp
index 523d98a93..333c9c229 100644
--- a/src/ast/ast_smt2_pp.cpp
+++ b/src/ast/ast_smt2_pp.cpp
@@ -364,16 +364,18 @@ format * smt2_pp_environment::pp_arith_literal(app * t, bool decimal, unsigned d
 }
 
 format * smt2_pp_environment::pp_string_literal(app * t) {
-    std::string s;
+    zstring s;
+    std::string encs;
     VERIFY (get_sutil().str.is_string(t, s));
+    encs = s.encode();
     std::ostringstream buffer;
     buffer << "\"";
-    for (unsigned i = 0; i < s.length(); ++i) {
-        if (s[i] == '\"') {
+    for (unsigned i = 0; i < encs.length(); ++i) {
+        if (encs[i] == '\"') {
             buffer << "\"\"";
         }
         else {
-            buffer << s[i];
+            buffer << encs[i];
         }
     }
     buffer << "\"";  
diff --git a/src/ast/rewriter/seq_rewriter.cpp b/src/ast/rewriter/seq_rewriter.cpp
index e64dacdf1..1eaf6b42b 100644
--- a/src/ast/rewriter/seq_rewriter.cpp
+++ b/src/ast/rewriter/seq_rewriter.cpp
@@ -121,7 +121,7 @@ br_status seq_rewriter::mk_app_core(func_decl * f, unsigned num_args, expr * con
    (a + string) + string = a + string
 */
 br_status seq_rewriter::mk_seq_concat(expr* a, expr* b, expr_ref& result) {
-    std::string s1, s2;
+    zstring s1, s2;
     expr* c, *d;
     bool isc1 = m_util.str.is_string(a, s1);
     bool isc2 = m_util.str.is_string(b, s2);
@@ -150,10 +150,10 @@ br_status seq_rewriter::mk_seq_concat(expr* a, expr* b, expr_ref& result) {
 }
 
 br_status seq_rewriter::mk_seq_length(expr* a, expr_ref& result) {
-    std::string b;
+    zstring b;
     m_es.reset();
     m_util.str.get_concat(a, m_es);
-    size_t len = 0;
+    unsigned len = 0;
     unsigned j = 0;
     for (unsigned i = 0; i < m_es.size(); ++i) {
         if (m_util.str.is_string(m_es[i], b)) {
@@ -189,21 +189,21 @@ br_status seq_rewriter::mk_seq_length(expr* a, expr_ref& result) {
 }
 
 br_status seq_rewriter::mk_seq_extract(expr* a, expr* b, expr* c, expr_ref& result) {
-    std::string s;
+    zstring s;
     rational pos, len;
     if (m_util.str.is_string(a, s) && m_autil.is_numeral(b, pos) && m_autil.is_numeral(c, len) &&
         pos.is_unsigned() && len.is_unsigned() && pos.get_unsigned() <= s.length()) {
         unsigned _pos = pos.get_unsigned();
         unsigned _len = len.get_unsigned();
-        result = m_util.str.mk_string(s.substr(_pos, _len));
+        result = m_util.str.mk_string(s.extract(_pos, _len));
         return BR_DONE;
     }
     return BR_FAILED;
 }
 br_status seq_rewriter::mk_seq_contains(expr* a, expr* b, expr_ref& result) {
-    std::string c, d;
+    zstring c, d;
     if (m_util.str.is_string(a, c) && m_util.str.is_string(b, d)) {
-        result = m().mk_bool_val(0 != strstr(c.c_str(), d.c_str()));
+        result = m().mk_bool_val(c.contains(d));
         return BR_DONE;
     }
     // check if subsequence of b is in a.
@@ -225,15 +225,12 @@ br_status seq_rewriter::mk_seq_contains(expr* a, expr* b, expr_ref& result) {
 }
 
 br_status seq_rewriter::mk_seq_at(expr* a, expr* b, expr_ref& result) {
-    std::string c;
+    zstring c;
     rational r;
     if (m_util.str.is_string(a, c) && m_autil.is_numeral(b, r) && r.is_unsigned()) {
         unsigned j = r.get_unsigned();
         if (j < c.length()) {
-            char ch = c[j];
-            c[0] = ch;
-            c[1] = 0;
-            result = m_util.str.mk_string(c);
+            result = m_util.str.mk_string(c.extract(j, 1));
             return BR_DONE;
         }
     }
@@ -241,19 +238,14 @@ br_status seq_rewriter::mk_seq_at(expr* a, expr* b, expr_ref& result) {
 }
 
 br_status seq_rewriter::mk_seq_index(expr* a, expr* b, expr* c, expr_ref& result) {
-    std::string s1, s2;
+    zstring s1, s2;
     rational r;
     bool isc1 = m_util.str.is_string(a, s1);
     bool isc2 = m_util.str.is_string(b, s2);
 
     if (isc1 && isc2 && m_autil.is_numeral(c, r) && r.is_unsigned()) {
-        for (unsigned i = r.get_unsigned(); i < s1.length(); ++i) {
-            if (strncmp(s1.c_str() + i, s2.c_str(), s2.length()) == 0) {
-                result = m_autil.mk_numeral(rational(i) - r, true);
-                return BR_DONE;
-            }
-        }
-        result = m_autil.mk_numeral(rational(-1), true);
+        int idx = s1.indexof(s2, r.get_unsigned());
+        result = m_autil.mk_numeral(rational(idx), true);
         return BR_DONE;
     }
     if (m_autil.is_numeral(c, r) && r.is_neg()) {
@@ -270,23 +262,10 @@ br_status seq_rewriter::mk_seq_index(expr* a, expr* b, expr* c, expr_ref& result
 }
 
 br_status seq_rewriter::mk_seq_replace(expr* a, expr* b, expr* c, expr_ref& result) {
-    std::string s1, s2, s3;
+    zstring s1, s2, s3;
     if (m_util.str.is_string(a, s1) && m_util.str.is_string(b, s2) && 
         m_util.str.is_string(c, s3)) {
-        std::ostringstream buffer;
-        bool can_replace = true;
-        for (size_t i = 0; i < s1.length(); ) {
-            if (can_replace && strncmp(s1.c_str() + i, s2.c_str(), s2.length()) == 0) {
-                buffer << s3;
-                i += s2.length();
-                can_replace = false;
-            }
-            else {
-                buffer << s1[i];
-                ++i;
-            }
-        }
-        result = m_util.str.mk_string(buffer.str());
+        result = m_util.str.mk_string(s1.replace(s2, s3));
         return BR_DONE;
     }
     if (b == c) {
@@ -298,15 +277,11 @@ br_status seq_rewriter::mk_seq_replace(expr* a, expr* b, expr* c, expr_ref& resu
 
 br_status seq_rewriter::mk_seq_prefix(expr* a, expr* b, expr_ref& result) {
     TRACE("seq", tout << mk_pp(a, m()) << " " << mk_pp(b, m()) << "\n";);
-    std::string s1, s2;
+    zstring s1, s2;
     bool isc1 = m_util.str.is_string(a, s1);
     bool isc2 = m_util.str.is_string(b, s2);
     if (isc1 && isc2) {
-        bool prefix = s1.length() <= s2.length();
-        for (unsigned i = 0; i < s1.length() && prefix; ++i) {
-            prefix = s1[i] == s2[i];
-        }
-        result = m().mk_bool_val(prefix);
+        result = m().mk_bool_val(s1.prefixof(s2));
         return BR_DONE;
     }
     if (m_util.str.is_empty(a)) {
@@ -321,7 +296,7 @@ br_status seq_rewriter::mk_seq_prefix(expr* a, expr* b, expr_ref& result) {
 
     if (a1 != b1 && isc1 && isc2) {
         if (s1.length() <= s2.length()) {
-            if (strncmp(s1.c_str(), s2.c_str(), s1.length()) == 0) {
+            if (s1.prefixof(s2)) {
                 if (a == a1) {
                     result = m().mk_true();
                     return BR_DONE;
@@ -329,10 +304,10 @@ br_status seq_rewriter::mk_seq_prefix(expr* a, expr* b, expr_ref& result) {
                 m_util.str.get_concat(a, as);
                 m_util.str.get_concat(b, bs);
                 SASSERT(as.size() > 1);
-                s2 = std::string(s2.c_str() + s1.length(), s2.length() - s1.length());
+                s2 = s2.extract(s1.length(), s2.length()-s1.length());
                 bs[0] = m_util.str.mk_string(s2);
                 result = m_util.str.mk_prefix(m_util.str.mk_concat(as.size()-1, as.c_ptr()+1),
-                                     m_util.str.mk_concat(bs.size(), bs.c_ptr()));
+                                              m_util.str.mk_concat(bs.size(), bs.c_ptr()));
                 return BR_REWRITE_FULL;
             }
             else {
@@ -341,7 +316,7 @@ br_status seq_rewriter::mk_seq_prefix(expr* a, expr* b, expr_ref& result) {
             }
         }
         else {
-            if (strncmp(s1.c_str(), s2.c_str(), s2.length()) == 0) {
+            if (s2.prefixof(s1)) {
                 if (b == b1) {
                     result = m().mk_false();
                     return BR_DONE;
@@ -349,7 +324,7 @@ br_status seq_rewriter::mk_seq_prefix(expr* a, expr* b, expr_ref& result) {
                 m_util.str.get_concat(a, as);
                 m_util.str.get_concat(b, bs);
                 SASSERT(bs.size() > 1);
-                s1 = std::string(s1.c_str() + s2.length(), s1.length() - s2.length());
+                s1 = s1.extract(s2.length(), s1.length() - s2.length());
                 as[0] = m_util.str.mk_string(s1);
                 result = m_util.str.mk_prefix(m_util.str.mk_concat(as.size(), as.c_ptr()),
                                      m_util.str.mk_concat(bs.size()-1, bs.c_ptr()+1));
@@ -396,7 +371,7 @@ br_status seq_rewriter::mk_seq_suffix(expr* a, expr* b, expr_ref& result) {
         result = m().mk_true();
         return BR_DONE;
     }
-    std::string s1, s2;
+    zstring s1, s2;
     if (m_util.str.is_empty(a)) {
         result = m().mk_true();
         return BR_DONE;
@@ -438,21 +413,18 @@ br_status seq_rewriter::mk_seq_suffix(expr* a, expr* b, expr_ref& result) {
     }
     if (isc1 && isc2) {
         if (s1.length() == s2.length()) {
-            SASSERT(s1 != s2);
+            //SASSERT(s1 != s2);
             result = m().mk_false();
             return BR_DONE;
         }
         else if (s1.length() < s2.length()) {
-            bool suffix = true;
-            for (unsigned i = 0; i < s1.length(); ++i) {
-                suffix = s1[s1.length()-i-1] == s2[s2.length()-i-1];
-            }
+            bool suffix = s1.suffixof(s2);
             if (suffix && a1 == 0) {
                 result = m().mk_true();
                 return BR_DONE;
             }
             else if (suffix) {
-                s2 = std::string(s2.c_str(), s2.length()-s1.length());
+                s2 = s2.extract(0, s2.length()-s1.length());
                 b2 = m_util.str.mk_string(s2);
                 result = m_util.str.mk_suffix(a1, b1?m_util.str.mk_concat(b1, b2):b2);
                 return BR_DONE;
@@ -468,12 +440,9 @@ br_status seq_rewriter::mk_seq_suffix(expr* a, expr* b, expr_ref& result) {
                 result = m().mk_false();
                 return BR_DONE;
             }
-            bool suffix = true;
-            for (unsigned i = 0; i < s2.length(); ++i) {
-                suffix = s1[s1.length()-i-1] == s2[s2.length()-i-1];
-            }
+            bool suffix = s2.suffixof(s1);
             if (suffix) {
-                s1 = std::string(s1.c_str(), s1.length()-s2.length());
+                s1 = s1.extract(0, s1.length()-s2.length());
                 a2 = m_util.str.mk_string(s1);
                 result = m_util.str.mk_suffix(a1?m_util.str.mk_concat(a1, a2):a2, b1);
                 return BR_DONE;
@@ -491,14 +460,15 @@ br_status seq_rewriter::mk_seq_suffix(expr* a, expr* b, expr_ref& result) {
 br_status seq_rewriter::mk_str_itos(expr* a, expr_ref& result) {
     rational r;
     if (m_autil.is_numeral(a, r)) {
-        result = m_util.str.mk_string(r.to_string());
+        result = m_util.str.mk_string(symbol(r.to_string().c_str()));
         return BR_DONE;
     }
     return BR_FAILED;
 }
 br_status seq_rewriter::mk_str_stoi(expr* a, expr_ref& result) {
-    std::string s;
-    if (m_util.str.is_string(a, s)) {
+    zstring str;
+    if (m_util.str.is_string(a, str)) {
+        std::string s = str.encode();
         for (unsigned i = 0; i < s.length(); ++i) {
             if (s[i] == '-') { if (i != 0) return BR_FAILED; }
             else if ('0' <= s[i] && s[i] <= '9') continue;
@@ -608,13 +578,13 @@ bool seq_rewriter::reduce_eq(expr* l, expr* r, expr_ref_vector& lhs, expr_ref_ve
         change = true;
     }
     // reduce strings
-    std::string s1, s2;
+    zstring s1, s2;
     while (head1 < m_lhs.size() && 
            head2 < m_rhs.size() && 
            m_util.str.is_string(m_lhs[head1], s1) &&
            m_util.str.is_string(m_rhs[head2], s2)) {
-        size_t l = std::min(s1.length(), s2.length());
-        for (size_t i = 0; i < l; ++i) {
+        unsigned l = std::min(s1.length(), s2.length());
+        for (unsigned i = 0; i < l; ++i) {
             if (s1[i] != s2[i]) {
                 return false;
             }
@@ -623,14 +593,14 @@ bool seq_rewriter::reduce_eq(expr* l, expr* r, expr_ref_vector& lhs, expr_ref_ve
             ++head1;            
         }
         else {
-            m_lhs[head1] = m_util.str.mk_string(std::string(s1.c_str()+l,s1.length()-l));
+            m_lhs[head1] = m_util.str.mk_string(s1.extract(l, s1.length()-l));
             trail.push_back(m_lhs[head1]);
         }
         if (l == s2.length()) {
             ++head2;            
         }
         else {
-            m_rhs[head2] = m_util.str.mk_string(std::string(s2.c_str()+l,s2.length()-l));
+            m_rhs[head2] = m_util.str.mk_string(s2.extract(l, s2.length()-l));
             trail.push_back(m_rhs[head2]);
         }
         change = true;
@@ -639,8 +609,8 @@ bool seq_rewriter::reduce_eq(expr* l, expr* r, expr_ref_vector& lhs, expr_ref_ve
            head2 < m_rhs.size() &&
            m_util.str.is_string(m_lhs.back(), s1) &&
            m_util.str.is_string(m_rhs.back(), s2)) {
-        size_t l = std::min(s1.length(), s2.length());
-        for (size_t i = 0; i < l; ++i) {
+        unsigned l = std::min(s1.length(), s2.length());
+        for (unsigned i = 0; i < l; ++i) {
             if (s1[s1.length()-i-1] != s2[s2.length()-i-1]) {
                 return false;
             }
@@ -648,11 +618,11 @@ bool seq_rewriter::reduce_eq(expr* l, expr* r, expr_ref_vector& lhs, expr_ref_ve
         m_lhs.pop_back();          
         m_rhs.pop_back();
         if (l < s1.length()) {
-            m_lhs.push_back(m_util.str.mk_string(std::string(s1.c_str(),s1.length()-l)));
+            m_lhs.push_back(m_util.str.mk_string(s1.extract(0, s1.length()-l)));
             trail.push_back(m_lhs.back());
         }
         if (l < s2.length()) {
-            m_rhs.push_back(m_util.str.mk_string(std::string(s2.c_str(),s2.length()-l)));
+            m_rhs.push_back(m_util.str.mk_string(s2.extract(0, s2.length()-l)));
             trail.push_back(m_rhs.back());
         }
         change = true;
@@ -703,7 +673,7 @@ expr* seq_rewriter::concat_non_empty(unsigned n, expr* const* as) {
 }
 
 bool seq_rewriter::set_empty(unsigned sz, expr* const* es, bool all, expr_ref_vector& lhs, expr_ref_vector& rhs) {
-    std::string s;
+    zstring s;
     for (unsigned i = 0; i < sz; ++i) {
         if (m_util.str.is_unit(es[i])) {
             if (all) return false;
@@ -725,8 +695,8 @@ bool seq_rewriter::set_empty(unsigned sz, expr* const* es, bool all, expr_ref_ve
     return true;
 }
 
-bool seq_rewriter::min_length(unsigned n, expr* const* es, size_t& len) {
-    std::string s;
+bool seq_rewriter::min_length(unsigned n, expr* const* es, unsigned& len) {
+    zstring s;
     bool bounded = true;
     len = 0;
     for (unsigned i = 0; i < n; ++i) {
@@ -749,7 +719,7 @@ bool seq_rewriter::min_length(unsigned n, expr* const* es, size_t& len) {
 bool seq_rewriter::length_constrained(unsigned szl, expr* const* l, unsigned szr, expr* const* r, 
                                       expr_ref_vector& lhs, expr_ref_vector& rhs, bool& is_sat) {
     is_sat = true;
-    size_t len1 = 0, len2 = 0;
+    unsigned len1 = 0, len2 = 0;
     bool bounded1 = min_length(szl, l, len1);
     bool bounded2 = min_length(szr, r, len2);
     if (bounded1 && len1 < len2) {
diff --git a/src/ast/rewriter/seq_rewriter.h b/src/ast/rewriter/seq_rewriter.h
index 998b37d1a..de3634a51 100644
--- a/src/ast/rewriter/seq_rewriter.h
+++ b/src/ast/rewriter/seq_rewriter.h
@@ -58,7 +58,7 @@ class seq_rewriter {
                         expr_ref_vector& lhs, expr_ref_vector& rhs, bool& is_sat);
     bool length_constrained(unsigned n, expr* const* l, unsigned m, expr* const* r, 
                         expr_ref_vector& lhs, expr_ref_vector& rhs, bool& is_sat);
-    bool min_length(unsigned n, expr* const* es, size_t& len);
+    bool min_length(unsigned n, expr* const* es, unsigned& len);
     expr* concat_non_empty(unsigned n, expr* const* es);
 
 public:    
diff --git a/src/ast/seq_decl_plugin.cpp b/src/ast/seq_decl_plugin.cpp
index 49b1873d9..e33da18f2 100644
--- a/src/ast/seq_decl_plugin.cpp
+++ b/src/ast/seq_decl_plugin.cpp
@@ -22,6 +22,145 @@ Revision History:
 #include "ast_pp.h"
 #include <sstream>
 
+zstring::zstring(encoding enc): m_encoding(enc) {}
+
+zstring::zstring(char const* s, encoding enc): m_encoding(enc) {
+    // TBD: epply decoding
+    while (*s) {
+        m_buffer.push_back(*s);
+        ++s;
+    }
+}
+
+zstring::zstring(zstring const& other) {
+    m_buffer = other.m_buffer;
+    m_encoding = other.m_encoding;
+}
+
+zstring::zstring(unsigned num_bits, bool const* ch) {
+    SASSERT(num_bits == 8 || num_bits == 16);
+    m_encoding = (num_bits == 8)?ascii:unicode;
+    unsigned n = 0;    
+    for (unsigned i = 0; i < num_bits; ++i) {
+        n |= (((unsigned)ch[i]) << num_bits);
+    }
+    m_buffer.push_back(n);
+}
+
+zstring::zstring(unsigned ch, encoding enc) {
+    m_encoding = enc;
+    m_buffer.push_back(ch & ((enc == ascii)?0x000000FF:0x0000FFFF));
+}
+
+zstring& zstring::operator=(zstring const& other) {
+    m_encoding = other.m_encoding;
+    m_buffer.reset();
+    m_buffer.append(other.m_buffer);
+    return *this;
+}
+
+zstring zstring::replace(zstring const& src, zstring const& dst) const {
+    zstring result(m_encoding);
+    if (length() < src.length()) {
+        return zstring(*this);
+    }
+    bool found = false;
+    for (unsigned i = 0; i <= length() - src.length(); ++i) {
+        bool eq = !found;
+        for (unsigned j = 0; eq && j < src.length(); ++j) {
+            eq = m_buffer[i+j] == src[j];
+        }
+        if (eq) {
+            result.m_buffer.append(dst.m_buffer);
+            found = true;
+        }
+        else {
+            result.m_buffer.push_back(m_buffer[i]);
+        }
+    }
+    return result;
+}
+
+std::string zstring::encode() const {
+    // TBD apply encodings.
+    SASSERT(m_encoding == ascii);
+    std::ostringstream strm;
+    for (unsigned i = 0; i < m_buffer.size(); ++i) {
+        strm << (char)(m_buffer[i]);
+    }
+    return strm.str();
+}
+
+bool zstring::suffixof(zstring const& other) const {
+    if (length() > other.length()) return false;
+    bool suffix = true;
+    for (unsigned i = 0; suffix && i < length(); ++i) {
+        suffix = m_buffer[length()-i-1] == other[other.length()-i-1];
+    }
+    return suffix;
+}
+
+bool zstring::prefixof(zstring const& other) const {
+    if (length() > other.length()) return false;
+    bool prefix = true;
+    for (unsigned i = 0; prefix && i < length(); ++i) {
+        prefix = m_buffer[i] == other[i];
+    }
+    return prefix;
+}
+
+bool zstring::contains(zstring const& other) const {
+    if (other.length() > length()) return false;
+    unsigned last = length() - other.length();
+    bool cont = false;
+    for (unsigned i = 0; !cont && i <= last; ++i) {
+        cont = true;
+        for (unsigned j = 0; cont && j < other.length(); ++j) {
+            cont = other[j] == m_buffer[j+i];
+        }
+    }
+    return cont;
+}
+
+int zstring::indexof(zstring const& other, int offset) const {
+    SASSERT(offset >= 0);
+    if (offset == length()) return -1;
+    if (other.length() + offset > length()) return -1;
+    unsigned last = length() - other.length();
+    for (unsigned i = static_cast<unsigned>(offset); i <= last; ++i) {
+        bool prefix = true;
+        for (unsigned j = 0; prefix && j < other.length(); ++j) {
+            prefix = m_buffer[i + j] == other[j];
+        }    
+        if (prefix) {
+            return static_cast<int>(i);
+        }
+    }
+    return -1;
+}
+
+zstring zstring::extract(int offset, int len) const {
+    zstring result(m_encoding);
+    SASSERT(0 <= offset && 0 <= len);
+    int last = std::min(offset+len, static_cast<int>(length()));
+    for (int i = offset; i < last; ++i) {
+        result.m_buffer.push_back(m_buffer[i]);
+    }
+    return result;
+}
+
+zstring zstring::operator+(zstring const& other) const {
+    SASSERT(m_encoding == other.m_encoding);
+    zstring result(*this);
+    result.m_buffer.append(other.m_buffer);
+    return result;
+}
+
+std::ostream& zstring::operator<<(std::ostream& out) const {
+    return out << encode();
+}
+
+
 seq_decl_plugin::seq_decl_plugin(): m_init(false), 
                                     m_stringc_sym("String"),
                                     m_string(0),
@@ -452,6 +591,14 @@ app* seq_decl_plugin::mk_string(symbol const& s) {
     return m_manager->mk_const(f);
 }
 
+app* seq_decl_plugin::mk_string(zstring const& s) {
+    symbol sym(s.encode().c_str());
+    parameter param(sym);
+    func_decl* f = m_manager->mk_const_decl(m_stringc_sym, m_string,
+                                   func_decl_info(m_family_id, OP_STRING_CONST, 1, &param));
+    return m_manager->mk_const(f);
+}
+
 bool seq_decl_plugin::is_value(app* e) const {
     return is_app_of(e, m_family_id, OP_STRING_CONST);
 }
@@ -463,6 +610,18 @@ app* seq_util::mk_skolem(symbol const& name, unsigned n, expr* const* args, sort
     return m.mk_app(f, n, args);
 }
 
+app* seq_util::str::mk_string(zstring const& s) { return u.seq.mk_string(s); }
+
+bool seq_util::str::is_string(expr const* n, zstring& s) const {
+    if (is_string(n)) {
+        s = zstring(to_app(n)->get_decl()->get_parameter(0).get_symbol().bare_str());
+        return true;
+    }
+    else {
+        return false;
+    }
+}
+
 
 void seq_util::str::get_concat(expr* e, ptr_vector<expr>& es) const {
     expr* e1, *e2;
diff --git a/src/ast/seq_decl_plugin.h b/src/ast/seq_decl_plugin.h
index 7026a73cb..156bf4735 100644
--- a/src/ast/seq_decl_plugin.h
+++ b/src/ast/seq_decl_plugin.h
@@ -81,6 +81,36 @@ enum seq_op_kind {
 };
 
 
+class zstring {
+public:
+    enum encoding {
+        ascii, 
+        unicode
+    };
+private:    
+    buffer<unsigned> m_buffer;
+    encoding         m_encoding;
+public:
+    zstring(encoding enc = ascii);
+    zstring(char const* s, encoding enc = ascii);
+    zstring(zstring const& other);
+    zstring(unsigned num_bits, bool const* ch);
+    zstring(unsigned ch, encoding enc = ascii);
+    zstring& operator=(zstring const& other);
+    zstring replace(zstring const& src, zstring const& dst) const;
+    unsigned num_bits() const { return (m_encoding==ascii)?8:16; }
+    std::string encode() const; 
+    unsigned length() const { return m_buffer.size(); }
+    unsigned operator[](unsigned i) const { return m_buffer[i]; }
+    bool empty() const { return m_buffer.empty(); }
+    bool suffixof(zstring const& other) const;
+    bool prefixof(zstring const& other) const;
+    bool contains(zstring const& other) const;
+    int  indexof(zstring const& other, int offset) const;
+    zstring extract(int lo, int hi) const;
+    zstring operator+(zstring const& other) const;
+    std::ostream& operator<<(std::ostream& out) const;
+};
     
 class seq_decl_plugin : public decl_plugin {
     struct psig {
@@ -146,6 +176,7 @@ public:
     bool is_char(ast* a) const { return a == m_char; }
 
     app* mk_string(symbol const& s);  
+    app* mk_string(zstring const& s);  
 };
 
 class seq_util {
@@ -171,14 +202,18 @@ public:
         seq_util&    u;
         ast_manager& m;
         family_id    m_fid;
+
+        app* mk_string(char const* s) { return mk_string(symbol(s)); }
+        app* mk_string(std::string const& s) { return mk_string(symbol(s.c_str())); }
+
+
     public:
         str(seq_util& u): u(u), m(u.m), m_fid(u.m_fid) {}
 
         sort* mk_seq(sort* s) { parameter param(s); return m.mk_sort(m_fid, SEQ_SORT, 1, &param); }
         app* mk_empty(sort* s) { return m.mk_const(m.mk_func_decl(m_fid, OP_SEQ_EMPTY, 0, 0, 0, (expr*const*)0, s)); }
+        app* mk_string(zstring const& s);
         app* mk_string(symbol const& s) { return u.seq.mk_string(s); }
-        app* mk_string(char const* s) { return mk_string(symbol(s)); }
-        app* mk_string(std::string const& s) { return mk_string(symbol(s.c_str())); }
         app* mk_concat(expr* a, expr* b) { expr* es[2] = { a, b }; return m.mk_app(m_fid, OP_SEQ_CONCAT, 2, es); }
         app* mk_concat(expr* a, expr* b, expr* c) {
             return mk_concat(mk_concat(a, b), c);
@@ -193,14 +228,13 @@ public:
 
 
         bool is_string(expr const * n) const { return is_app_of(n, m_fid, OP_STRING_CONST); }
-        
-        bool is_string(expr const* n, std::string& s) const {
-            return is_string(n) && (s = to_app(n)->get_decl()->get_parameter(0).get_symbol().str(), true);
-        }
+
         bool is_string(expr const* n, symbol& s) const {
             return is_string(n) && (s = to_app(n)->get_decl()->get_parameter(0).get_symbol(), true);
         }
         
+        bool is_string(expr const* n, zstring& s) const;
+
         bool is_empty(expr const* n) const { symbol s; 
             return is_app_of(n, m_fid, OP_SEQ_EMPTY) || (is_string(n, s) && !s.is_numerical() && *s.bare_str() == 0); 
         }
diff --git a/src/parsers/smt2/smt2parser.cpp b/src/parsers/smt2/smt2parser.cpp
index 404c04d3a..c6736c1e0 100644
--- a/src/parsers/smt2/smt2parser.cpp
+++ b/src/parsers/smt2/smt2parser.cpp
@@ -1069,7 +1069,7 @@ namespace smt2 {
 
         void parse_string_const() {
             SASSERT(curr() == scanner::STRING_TOKEN);
-            expr_stack().push_back(sutil().str.mk_string(m_scanner.get_string()));
+            expr_stack().push_back(sutil().str.mk_string(symbol(m_scanner.get_string())));
             TRACE("smt2parser", tout << "new string: " << mk_pp(expr_stack().back(), m()) << "\n";);
             next();
         }
diff --git a/src/smt/smt_setup.cpp b/src/smt/smt_setup.cpp
index 8ebfa2d71..8a40f9d7a 100644
--- a/src/smt/smt_setup.cpp
+++ b/src/smt/smt_setup.cpp
@@ -815,7 +815,7 @@ namespace smt {
     }
 
     void setup::setup_seq() {
-        m_context.register_plugin(alloc(theory_seq_empty, m_manager));
+        m_context.register_plugin(alloc(theory_seq, m_manager));
     }
 
     void setup::setup_card() {
diff --git a/src/smt/theory_seq.cpp b/src/smt/theory_seq.cpp
index f0b82adbb..67ab2cfe3 100644
--- a/src/smt/theory_seq.cpp
+++ b/src/smt/theory_seq.cpp
@@ -127,6 +127,7 @@ theory_seq::theory_seq(ast_manager& m):
     m(m),
     m_dam(m_dep_array_value_manager, m_alloc),
     m_rep(m, m_dm),
+    m_cache(m),
     m_sort2len_fn(m),
     m_factory(0),
     m_ineqs(m),
@@ -135,6 +136,7 @@ theory_seq::theory_seq(ast_manager& m):
     m_axioms_head(0),
     m_branch_variable_head(0),
     m_incomplete(false), 
+    m_has_length(false),
     m_model_completion(false),
     m_rewrite(m), 
     m_util(m),
@@ -249,10 +251,10 @@ bool theory_seq::find_branch_candidate(expr* l, ptr_vector<expr> const& rs) {
         if (occurs(l, rs[j])) {
             return false;
         }
-        std::string s;
+        zstring s;
         if (m_util.str.is_string(rs[j], s)) {
             for (size_t k = 1; k < s.length(); ++k) {
-                v = m_util.str.mk_string(std::string(s.c_str(), k));
+                v = m_util.str.mk_string(s.extract(0, k));
                 if (v0) v = m_util.str.mk_concat(v0, v);
                 if (assume_equality(l, v)) {
                     return true;
@@ -426,6 +428,7 @@ bool theory_seq::is_right_select(expr* a, expr*& b) {
 void theory_seq::add_solution(expr* l, expr* r, enode_pair_dependency* deps) {
     context& ctx = get_context();
     m_rep.update(l, r, deps);
+    m_cache.reset();
     // TBD: skip new equalities for non-internalized terms.
     if (ctx.e_internalized(l) && ctx.e_internalized(r)) {
         propagate_eq(deps, ctx.get_enode(l), ctx.get_enode(r));
@@ -508,6 +511,10 @@ bool theory_seq::internalize_term(app* term) {
         } 
         mk_var(e);
     }
+    if (m_util.str.is_length(term) && !m_has_length) {
+        m_trail_stack.push(value_trail<theory_seq, bool>(m_has_length)); 
+        m_has_length = true; 
+    }
     if (!m_util.str.is_concat(term) &&
         !m_util.str.is_string(term) &&
         !m_util.str.is_empty(term)  && 
@@ -635,37 +642,49 @@ expr_ref theory_seq::canonize(expr* e, enode_pair_dependency*& eqs) {
 
 expr_ref theory_seq::expand(expr* e, enode_pair_dependency*& eqs) {
     enode_pair_dependency* deps = 0;
+    expr* r = 0;
+    if (m_cache.find(e, r)) {
+        return expr_ref(r, m);
+    }
     e = m_rep.find(e, deps);
+    expr_ref result(m);
     expr* e1, *e2;
     eqs = m_dm.mk_join(eqs, deps);
     if (m_util.str.is_concat(e, e1, e2)) {
-        return expr_ref(m_util.str.mk_concat(expand(e1, eqs), expand(e2, eqs)), m);
+        result = m_util.str.mk_concat(expand(e1, eqs), expand(e2, eqs));
     }        
-    if (m_util.str.is_empty(e) || m_util.str.is_string(e)) {
-        return expr_ref(e, m);
+    else if (m_util.str.is_empty(e) || m_util.str.is_string(e)) {
+        result = e;
     }
-    if (m.is_eq(e, e1, e2)) {
-        return expr_ref(m.mk_eq(expand(e1, eqs), expand(e2, eqs)), m);
+    else if (m.is_eq(e, e1, e2)) {
+        result = m.mk_eq(expand(e1, eqs), expand(e2, eqs));
     }
-    if (m_util.str.is_prefix(e, e1, e2)) {
-        return expr_ref(m_util.str.mk_prefix(expand(e1, eqs), expand(e2, eqs)), m);
+    else if (m_util.str.is_prefix(e, e1, e2)) {
+        result = m_util.str.mk_prefix(expand(e1, eqs), expand(e2, eqs));
     }
-    if (m_util.str.is_suffix(e, e1, e2)) {
-        return expr_ref(m_util.str.mk_suffix(expand(e1, eqs), expand(e2, eqs)), m);
+    else if (m_util.str.is_suffix(e, e1, e2)) {
+        result = m_util.str.mk_suffix(expand(e1, eqs), expand(e2, eqs));
     }
-    if (m_util.str.is_contains(e, e1, e2)) {
-        return expr_ref(m_util.str.mk_contains(expand(e1, eqs), expand(e2, eqs)), m);
+    else if (m_util.str.is_contains(e, e1, e2)) {
+        result = m_util.str.mk_contains(expand(e1, eqs), expand(e2, eqs));
     }
-    if (m_model_completion && is_var(e)) {
+    else if (m_model_completion && is_var(e)) {
         SASSERT(m_factory);
         expr_ref val(m);
         val = m_factory->get_fresh_value(m.get_sort(e));
         if (val) {
             m_rep.update(e, val, 0);
-            return val;
+            result = val;
+        }
+        else {
+            result = e;
         }
     }
-    return expr_ref(e, m);
+    else {
+        result = e;
+    }
+    m_cache.insert(e, result);
+    return result;
 }
 
 void theory_seq::add_dependency(enode_pair_dependency*& dep, enode* a, enode* b) {
@@ -707,48 +726,18 @@ void theory_seq::deque_axiom(expr* n) {
     else if (m_util.str.is_at(n)) {
         add_at_axiom(n);
     }
-}
-
-/*
-  \brief nodes n1 and n2 are about to get merged.
-  if n1 occurs in the context of a length application,
-  then instantiate length axioms for each concatenation in the class of n2.
-  In this way we ensure that length respects concatenation.
- */
-void theory_seq::new_eq_len_concat(enode* n1, enode* n2) {
-    context& ctx = get_context();
-    if (n1->get_root() == n2->get_root()) {
-        return;
+    else if (m_util.str.is_unit(n)) {
+        add_length_unit_axiom(n);
     }
-    SASSERT(n1->get_root() != n2->get_root());
-    if (!m_util.is_seq(n1->get_owner())) {
-        return;
+    else if (m_util.str.is_empty(n)) {
+        add_length_empty_axiom(n);
     }
-    func_decl* f_len = 0;
-    if (!m_sort2len_fn.find(m.get_sort(n1->get_owner()), f_len)) {
-        return;
+    else if (m_util.str.is_concat(n)) {
+        add_length_concat_axiom(n);
     }
-
-    enode* r1 = n1->get_root();
-    enode_vector::const_iterator it  = ctx.begin_enodes_of(f_len);
-    enode_vector::const_iterator end = ctx.end_enodes_of(f_len);
-    bool has_len = false;
-    for (; !has_len && it != end; ++it) {
-        has_len = ((*it)->get_root() == r1);
+    else if (m_util.str.is_string(n)) {
+        add_length_string_axiom(n);
     }
-    if (!has_len) {
-        return;
-    }
-    enode* start2 = n2;
-    do {
-        expr* o = n2->get_owner();
-        if (!is_var(o)) {
-            expr_ref ln(m_util.str.mk_length(o), m);
-            enque_axiom(ln);
-        }
-        n2 = n2->get_next();
-    }
-    while (n2 != start2);
 }
 
 
@@ -760,7 +749,7 @@ void theory_seq::new_eq_len_concat(enode* n1, enode* n2) {
   lit or s = "" or len(c) = 1
   lit or s = "" or !prefix(s, x*s1)
 */
-void theory_seq::tightest_prefix(expr* s, expr* x, literal lit) {
+void theory_seq::tightest_prefix(expr* s, expr* x, literal lit1, literal lit2) {
     expr_ref s1 = mk_skolem(symbol("seq.first"), s);
     expr_ref c  = mk_skolem(symbol("seq.last"),  s);
     expr_ref s1c(m_util.str.mk_concat(s1, c), m);
@@ -768,26 +757,32 @@ void theory_seq::tightest_prefix(expr* s, expr* x, literal lit) {
     expr_ref one(m_autil.mk_int(1), m);
     expr_ref emp(m_util.str.mk_empty(m.get_sort(s)), m);
     literal s_eq_emp = mk_eq(s, emp, false);
-    add_axiom(lit, s_eq_emp, mk_eq(s, s1c, false));
-    add_axiom(lit, s_eq_emp, mk_eq(lc, one, false));
-    add_axiom(lit, s_eq_emp, ~mk_literal(m_util.str.mk_contains(s, m_util.str.mk_concat(x, s1))));
+    add_axiom(lit1, lit2, s_eq_emp, mk_eq(s, s1c, false));
+    add_axiom(lit1, lit2, s_eq_emp, mk_eq(lc, one, false));
+    add_axiom(lit1, lit2, s_eq_emp, ~mk_literal(m_util.str.mk_contains(s, m_util.str.mk_concat(x, s1))));
 }
 
 /*
+  // index of s in t starting at offset.
+
+  let i = Index(t, s, 0):
+  
+  len(t) = 0  => i = -1
+  len(t) != 0 & !contains(t, s) => i = -1
+  len(t) != 0 & contains(t, s) => t = xsy & i = len(x)
+  len(t) != 0 & contains(t, s) & s != emp => tightest_prefix(x, s)
+
   let i = Index(t, s, offset)
 
-  if offset = 0:
-    (!contains(t, s) -> i = -1)
-    (s = empty -> i = 0)
-    (contains(t, s) & s != empty -> t = xsy)
-    (contains(t, s) -> tightest_prefix(s, x))
-  if 0 <= offset < len(t):
-     t = zt' & len(z) == offset
-     add above constraints with t'
-  if offset >= len(t):
-     i = -1
-  if offset < 0:
-     ?
+
+  0 <= offset < len(t) => xy = t & len(x) = offset & (-1 = indexof(t, s, 0) => -1 = i)
+                                                   & (indexof(t, s, 0) >= 0 => indexof(t, s, 0) + offset = i)
+  
+
+  offset = len(t) => i = -1
+  
+  if offset < 0 or offset >= len(t)
+     under specified
 
   optional lemmas:
   (len(s) > len(t)  -> i = -1) 
@@ -801,20 +796,45 @@ void theory_seq::add_indexof_axiom(expr* i) {
     minus_one   = m_autil.mk_int(-1);
     zero        = m_autil.mk_int(0);
     emp         = m_util.str.mk_empty(m.get_sort(s));
-    if (m_autil.is_numeral(offset, r) && r.is_zero()) {
+    literal offset_ne_zero = null_literal;
+    bool is_num = m_autil.is_numeral(offset, r); 
+    if (is_num && r.is_zero()) {
+        offset_ne_zero = null_literal;
+    }
+    else {
+        offset_ne_zero = ~mk_eq(offset, zero, false);
+    }
+    if (!is_num || r.is_zero()) {
         expr_ref x  = mk_skolem(m_contains_left_sym, t, s);
         expr_ref y  = mk_skolem(m_contains_right_sym, t, s);    
         xsy         = m_util.str.mk_concat(x,s,y);
         literal cnt = mk_literal(m_util.str.mk_contains(t, s));
         literal eq_empty = mk_eq(s, emp, false);
-        add_axiom(cnt,  mk_eq(i, minus_one, false));
-        add_axiom(~eq_empty, mk_eq(i, zero, false));
-        add_axiom(~cnt, eq_empty, mk_eq(t, xsy, false));
-        tightest_prefix(s, x, ~cnt);
+        add_axiom(offset_ne_zero, cnt,  mk_eq(i, minus_one, false));
+        add_axiom(offset_ne_zero, ~eq_empty, mk_eq(i, zero, false));
+        add_axiom(offset_ne_zero, ~cnt, eq_empty, mk_eq(t, xsy, false));
+        tightest_prefix(s, x, ~cnt, offset_ne_zero);
     }
-    else {
-        // TBD
+    if (is_num && r.is_zero()) {
+        return;
     }
+    // offset >= len(t) => indexof(s, t, offset) = -1
+    expr_ref len_t(m_util.str.mk_length(t), m);
+    literal offset_ge_len = mk_literal(m_autil.mk_ge(mk_sub(offset, len_t), zero));
+    add_axiom(offset_ge_len, mk_eq(i, minus_one, false));
+
+    // 0 <= offset & offset < len(t) => t = xy
+    // 0 <= offset & offset < len(t) => len(x) = offset
+    // 0 <= offset & offset < len(t) & ~contains(s, y) => indexof(t, s, offset) = -1
+	// 0 <= offset & offset < len(t) & contains(s, y) => index(t, s, offset) = indexof(y, s, 0) + len(t) 
+    expr_ref x = mk_skolem(symbol("seq.indexof.left"), t, s, offset);
+    expr_ref y = mk_skolem(symbol("seq.indexof.right"), t, s, offset);
+    expr_ref indexof(m_util.str.mk_index(y, s, zero), m);
+	// TBD:
+    //literal offset_ge_0 = mk_literal(m_autil.mk_ge(offset, zero));
+    //add_axiom(~offset_ge_0, offset_ge_len, mk_eq(indexof, i, false));
+    //add_axiom(~offset_ge_0, offset_ge_len, mk_eq(m_util.str.mk_length(x), offset, false));
+    //add_axiom(~offset_ge_0, offset_ge_len, mk_eq(t, m_util.str.mk_concat(x, y), false));
 }
 
 /*
@@ -839,39 +859,56 @@ void theory_seq::add_replace_axiom(expr* r) {
     tightest_prefix(s, x, ~cnt);
 }
 
+void theory_seq::add_length_unit_axiom(expr* n) {
+    if (!m_has_length) return;
+    SASSERT(m_util.str.is_unit(n));
+    expr_ref one(m_autil.mk_int(1), m), len(m_util.str.mk_length(n), m);
+    add_axiom(mk_eq(len, one, false));
+}
+
+void theory_seq::add_length_empty_axiom(expr* n) {
+    if (!m_has_length) return;
+    SASSERT(m_util.str.is_empty(n));
+    expr_ref zero(m_autil.mk_int(0), m), len(m_util.str.mk_length(n), m);
+    add_axiom(mk_eq(len, zero, false));
+}
+
+void theory_seq::add_length_string_axiom(expr* n) {
+    if (!m_has_length) return;
+    zstring s;
+    VERIFY(m_util.str.is_string(n, s));
+    expr_ref len(m_util.str.mk_length(n), m);
+    expr_ref ls(m_autil.mk_numeral(rational(s.length(), rational::ui64()), true), m);    
+    add_axiom(mk_eq(len, ls, false));
+}
+
+void theory_seq::add_length_concat_axiom(expr* n) {
+    if (!m_has_length) return;
+    expr* a, *b;
+    VERIFY(m_util.str.is_concat(n, a, b));
+    expr_ref len(m_util.str.mk_length(n), m);
+    expr_ref _a(m_util.str.mk_length(a), m);
+    expr_ref _b(m_util.str.mk_length(b), m);
+    expr_ref a_p_b(m_autil.mk_add(_a, _b), m);
+    add_axiom(mk_eq(len, a_p_b, false));
+}
+
 /*
     let n = len(x)
 
     len(x) >= 0
     len(x) = 0 => x = ""
     x = "" => len(x) = 0
-    len(x) = rewrite(len(x))
  */
 void theory_seq::add_length_axiom(expr* n) {
-    expr* x, *a, *b;
+    expr* x;
     VERIFY(m_util.str.is_length(n, x));
-    expr_ref zero(m), one(m), emp(m);
-    zero = m_autil.mk_int(0);
-    std::string s;
-    if (m_util.str.is_unit(n)) {
-        one = m_autil.mk_int(1);
-        add_axiom(mk_eq(n, one, false));
-    }
-    else if (m_util.str.is_empty(n)) {
-        add_axiom(mk_eq(n, zero, false));
-    }
-    else if (m_util.str.is_string(n, s)) {
-        expr_ref ls(m_autil.mk_numeral(rational(s.length(), rational::ui64()), true), m);
-        add_axiom(mk_eq(n, ls, false));
-    }
-    else if (m_util.str.is_concat(n, a, b)) {
-        expr_ref _a(m_util.str.mk_length(a), m);
-        expr_ref _b(m_util.str.mk_length(b), m);
-        expr_ref a_p_b(m_autil.mk_add(_a, _b), m);
-        add_axiom(mk_eq(n, a_p_b, false));
-    }
-    else {
-        emp = m_util.str.mk_empty(m.get_sort(x));
+    if (!m_util.str.is_unit(x) &&
+        !m_util.str.is_empty(x) &&
+        !m_util.str.is_string(x) &&
+        !m_util.str.is_concat(x)) {
+        expr_ref zero(m_autil.mk_int(0), m);
+        expr_ref emp(m_util.str.mk_empty(m.get_sort(x)), m);
         literal eq1(mk_eq(zero, n, false));
         literal eq2(mk_eq(x, emp, false));
         add_axiom(mk_literal(m_autil.mk_ge(n, zero)));
@@ -966,9 +1003,10 @@ void theory_seq::add_axiom(literal l1, literal l2, literal l3, literal l4) {
 }
 
 
-expr_ref theory_seq::mk_skolem(symbol const& name, expr* e1, expr* e2) {
-    expr* es[2] = { e1, e2 };
-    return expr_ref(m_util.mk_skolem(name, e2?2:1, es, m.get_sort(e1)), m);
+expr_ref theory_seq::mk_skolem(symbol const& name, expr* e1, expr* e2, expr* e3) {
+    expr* es[3] = { e1, e2, e3 };
+    unsigned len = e3?3:(e2?2:1);
+    return expr_ref(m_util.mk_skolem(name, len, es, m.get_sort(e1)), m);
 }
 
 void theory_seq::propagate_eq(bool_var v, expr* e1, expr* e2) {
@@ -1040,8 +1078,7 @@ void theory_seq::new_eq_eh(theory_var v1, theory_var v2) {
         m.push_back(m_rhs.back(), o2);
         m_dam.push_back(m_deps.back(), m_dm.mk_leaf(enode_pair(n1, n2)));
 
-        new_eq_len_concat(n1, n2);
-        new_eq_len_concat(n2, n1);
+        // add length-equal axiom?
     }
 }
 
@@ -1087,6 +1124,7 @@ void theory_seq::pop_scope_eh(unsigned num_scopes) {
         m_rhs.pop_back();
         m_deps.pop_back();
     }
+    m_cache.reset();
 }
 
 void theory_seq::restart_eh() {
@@ -1108,7 +1146,11 @@ void theory_seq::relevant_eh(app* n) {
         m_util.str.is_index(n)   ||
         m_util.str.is_replace(n) ||
         m_util.str.is_extract(n) ||
-        m_util.str.is_at(n)) {
+        m_util.str.is_at(n) ||
+        m_util.str.is_concat(n) ||
+        m_util.str.is_empty(n) ||
+        m_util.str.is_unit(n) ||
+        m_util.str.is_string(n)) {
         enque_axiom(n);
     }
 }
diff --git a/src/smt/theory_seq.h b/src/smt/theory_seq.h
index 72363772b..1bf0454a8 100644
--- a/src/smt/theory_seq.h
+++ b/src/smt/theory_seq.h
@@ -87,6 +87,16 @@ namespace smt {
             void display(std::ostream& out) const;            
         };
 
+        class eval_cache {
+            obj_map<expr, expr*> m_map;
+            expr_ref_vector      m_trail;
+        public:
+            eval_cache(ast_manager& m): m_trail(m) {}
+            bool find(expr* v, expr*& r) const { return m_map.find(v, r); }
+            void insert(expr* v, expr* r) { m_trail.push_back(v); m_trail.push_back(r); m_map.insert(v, r); }
+            void reset() { m_map.reset(); m_trail.reset(); }
+        };
+
         struct stats {
             stats() { reset(); }
             void reset() { memset(this, 0, sizeof(stats)); }
@@ -101,6 +111,7 @@ namespace smt {
         solution_map                        m_rep;        // unification representative.
         vector<expr_array>                  m_lhs, m_rhs; // persistent sets of equalities.
         vector<enode_pair_dependency_array> m_deps;       // persistent sets of dependencies.
+        eval_cache                          m_cache;
 
         ast2ast_trailmap<sort, func_decl>   m_sort2len_fn; // length functions per sort.
         seq_factory*    m_factory;               // value factory
@@ -110,6 +121,7 @@ namespace smt {
         unsigned        m_axioms_head;           // index of first axiom to add.
         unsigned        m_branch_variable_head;  // index of first equation to examine.
         bool            m_incomplete;            // is the solver (clearly) incomplete for the fragment.
+        bool            m_has_length;            // is length applied
         bool            m_model_completion;      // during model construction, invent values in canonizer
         th_rewriter     m_rewrite;
         seq_util        m_util;
@@ -185,15 +197,16 @@ namespace smt {
         void add_replace_axiom(expr* e);
         void add_extract_axiom(expr* e);
         void add_length_axiom(expr* n);
+        void add_length_unit_axiom(expr* n);
+        void add_length_empty_axiom(expr* n);
+        void add_length_concat_axiom(expr* n);
+        void add_length_string_axiom(expr* n);
         void add_at_axiom(expr* n);
         literal mk_literal(expr* n);
-        void tightest_prefix(expr* s, expr* x, literal lit);
+        void tightest_prefix(expr* s, expr* x, literal lit, literal lit2 = null_literal);
         expr* mk_sub(expr* a, expr* b);
 
-        void new_eq_len_concat(enode* n1, enode* n2);
-
-
-        expr_ref mk_skolem(symbol const& s, expr* e1, expr* e2 = 0);
+        expr_ref mk_skolem(symbol const& s, expr* e1, expr* e2 = 0, expr* e3 = 0);
 
         void set_incomplete(app* term);
 
diff --git a/src/smt/theory_seq_empty.h b/src/smt/theory_seq_empty.h
index f391dab40..58a0fa85f 100644
--- a/src/smt/theory_seq_empty.h
+++ b/src/smt/theory_seq_empty.h
@@ -71,8 +71,8 @@ namespace smt {
         }
         virtual bool get_some_values(sort* s, expr_ref& v1, expr_ref& v2) { 
             if (u.is_string(s)) {
-                v1 = u.str.mk_string("a");
-                v2 = u.str.mk_string("b");
+                v1 = u.str.mk_string(symbol("a"));
+                v2 = u.str.mk_string(symbol("b"));
                 return true;
             }
             NOT_IMPLEMENTED_YET();