diff --git a/src/ast/rewriter/seq_rewriter.cpp b/src/ast/rewriter/seq_rewriter.cpp
index fce29426c..cbccfeb5e 100644
--- a/src/ast/rewriter/seq_rewriter.cpp
+++ b/src/ast/rewriter/seq_rewriter.cpp
@@ -374,7 +374,7 @@ br_status seq_rewriter::mk_seq_length(expr* a, expr_ref& result) {
             es.push_back(m_autil.mk_numeral(rational(len, rational::ui64()), true));
         }
         result = m_autil.mk_add(es.size(), es.c_ptr());
-        return BR_DONE;
+        return BR_REWRITE2;
     }
     return BR_FAILED;
 }
@@ -1170,14 +1170,13 @@ bool seq_rewriter::reduce_eq(expr_ref_vector& ls, expr_ref_vector& rs, expr_ref_
                 rs.pop_back();
             }
             else {
-                expr_ref s2(m_util.str.mk_string(s.extract(0, s.length()-2)), m());
+                expr_ref s2(m_util.str.mk_string(s.extract(0, s.length()-1)), m());
                 rs[rs.size()-1] = s2;
             }
         }
         else {
             break;
         }
-        TRACE("seq", tout << "change back\n";);
         change = true;
         lchange = true;
     }
diff --git a/src/ast/seq_decl_plugin.h b/src/ast/seq_decl_plugin.h
index 611020a69..29ec98b07 100644
--- a/src/ast/seq_decl_plugin.h
+++ b/src/ast/seq_decl_plugin.h
@@ -268,6 +268,7 @@ public:
         MATCH_TERNARY(is_extract);
         MATCH_BINARY(is_contains);
         MATCH_BINARY(is_at);
+        MATCH_BINARY(is_index);
         MATCH_TERNARY(is_index);
         MATCH_TERNARY(is_replace);
         MATCH_BINARY(is_prefix);
diff --git a/src/smt/theory_seq.cpp b/src/smt/theory_seq.cpp
index 61c09b020..cc84388d3 100644
--- a/src/smt/theory_seq.cpp
+++ b/src/smt/theory_seq.cpp
@@ -244,9 +244,9 @@ bool theory_seq::branch_variable() {
         unsigned k = (i + start) % sz;
         eq const& e = m_eqs[k];
         unsigned id = e.id();
-        TRACE("seq", tout << e.ls() << " = " << e.rs() << "\n";);
 
         s = find_branch_start(2*id);
+        TRACE("seq", tout << s << " " << 2*id << ": " << e.ls() << " = " << e.rs() << "\n";);
         bool found = find_branch_candidate(s, e.dep(), e.ls(), e.rs());
         insert_branch_start(2*id, s);
         if (found) {
@@ -297,11 +297,14 @@ bool theory_seq::find_branch_candidate(unsigned& start, dependency* dep, expr_re
 
     expr_ref v0(m);
     v0 = m_util.str.mk_empty(m.get_sort(l));
-    if (can_be_equal(ls.size() - 1, ls.c_ptr() + 1, rs.size(), rs.c_ptr()) && l_false != assume_equality(l, v0)) {
-        TRACE("seq", tout << mk_pp(l, m) << " " << v0 << "\n";);
-        return true;
+    literal_vector lits;
+    if (can_be_equal(ls.size() - 1, ls.c_ptr() + 1, rs.size(), rs.c_ptr())) {
+        if (l_false != assume_equality(l, v0)) {
+            TRACE("seq", tout << mk_pp(l, m) << " " << v0 << "\n";);
+            return true;
+        }
+        lits.push_back(~mk_eq_empty(l));
     }
-//    start = 0;
     for (; start < rs.size(); ++start) {
         unsigned j = start;
         SASSERT(!m_util.str.is_concat(rs[j]));
@@ -325,11 +328,11 @@ bool theory_seq::find_branch_candidate(unsigned& start, dependency* dep, expr_re
         all_units &= m_util.str.is_unit(rs[j]);
     }
     if (all_units) {
-        literal_vector lits;
-        lits.push_back(~mk_eq_empty(l));
         for (unsigned i = 0; i < rs.size(); ++i) {
-            v0 = mk_concat(i + 1, rs.c_ptr());
-            lits.push_back(~mk_eq(l, v0, false));
+            if (can_be_equal(ls.size() - 1, ls.c_ptr() + 1, rs.size() - i - 1, rs.c_ptr() + i + 1)) {
+                v0 = mk_concat(i + 1, rs.c_ptr());
+                lits.push_back(~mk_eq(l, v0, false));
+            }
         }
         set_conflict(dep, lits);
         TRACE("seq", tout << mk_pp(l, m) << " " << v0 << "\n";);
@@ -506,11 +509,23 @@ expr_ref theory_seq::mk_nth(expr* s, expr* idx) {
 }
 
 expr_ref theory_seq::mk_last(expr* s) {
+    zstring str;
+    if (m_util.str.is_string(s, str) && str.length() > 0) {
+        return expr_ref(m_util.str.mk_char(str, str.length()-1), m);
+    }
     sort* char_sort = 0;
     VERIFY(m_util.is_seq(m.get_sort(s), char_sort));
     return mk_skolem(m_seq_last, s, 0, 0, char_sort);
 }
 
+expr_ref theory_seq::mk_first(expr* s) {
+    zstring str;
+    if (m_util.str.is_string(s, str) && str.length() > 0) {
+        return expr_ref(m_util.str.mk_string(str.extract(0, str.length()-1)), m);
+    }
+    return mk_skolem(m_seq_first, s);
+}
+
 
 void theory_seq::mk_decompose(expr* e, expr_ref& head, expr_ref& tail) {
     expr* e1, *e2;
@@ -819,7 +834,6 @@ bool theory_seq::solve_eqs(unsigned i) {
     bool change = false;
     for (; !ctx.inconsistent() && i < m_eqs.size(); ++i) {
         eq const& e = m_eqs[i];
-        TRACE("seq", tout << i << "\n";);
         if (solve_eq(e.ls(), e.rs(), e.dep())) {
             if (i + 1 != m_eqs.size()) {
                 eq e1 = m_eqs[m_eqs.size()-1];
@@ -1579,8 +1593,9 @@ void theory_seq::propagate() {
         ++m_axioms_head;
     }
     while (!m_replay.empty() && !ctx.inconsistent()) {
-        (*m_replay[m_replay.size()-1])(*this);
         TRACE("seq", tout << "replay at level: " << ctx.get_scope_level() << "\n";);
+        apply* app = m_replay[m_replay.size() - 1];
+        (*app)(*this);
         m_replay.pop_back();
     }
     if (m_new_solution) {
@@ -1632,7 +1647,7 @@ void theory_seq::deque_axiom(expr* n) {
   lit or s = "" or !prefix(s, x*s1)
 */
 void theory_seq::tightest_prefix(expr* s, expr* x, literal lit1, literal lit2) {
-    expr_ref s1 = mk_skolem(m_seq_first, s);
+    expr_ref s1 = mk_first(s);
     expr_ref c  = mk_last(s);
     expr_ref s1c = mk_concat(s1, m_util.str.mk_unit(c));
     literal s_eq_emp = mk_eq_empty(s);
@@ -1668,30 +1683,34 @@ void theory_seq::tightest_prefix(expr* s, expr* x, literal lit1, literal lit2) {
    (len(s) <= len(t) -> i <= len(t)-len(s))
 */
 void theory_seq::add_indexof_axiom(expr* i) {
-    expr* s, *t, *offset;
+    expr* s, *t, *offset = 0;
     rational r;
-    VERIFY(m_util.str.is_index(i, t, s, offset));
+    VERIFY(m_util.str.is_index(i, t, s) ||
+           m_util.str.is_index(i, t, s, offset));
     expr_ref minus_one(m_autil.mk_int(-1), m);
     expr_ref zero(m_autil.mk_int(0), m);
     expr_ref xsy(m);
 
-    // 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));
 
-    if (m_autil.is_numeral(offset, r) && r.is_zero()) {
+    if (!offset || (m_autil.is_numeral(offset, r) && r.is_zero())) {
         expr_ref x  = mk_skolem(m_contains_left, t, s);
         expr_ref y  = mk_skolem(m_contains_right, t, s);
-        xsy         = mk_concat(x,s,y);
+        xsy         = mk_concat(x, s, y);
         literal cnt = mk_literal(m_util.str.mk_contains(t, s));
         literal eq_empty = mk_eq_empty(s);
         add_axiom(cnt,  mk_eq(i, minus_one, false));
         add_axiom(~eq_empty, mk_eq(i, zero, false));
+        add_axiom(eq_empty, ~mk_eq_empty(t), mk_eq(i, minus_one, false));
         add_axiom(~cnt, eq_empty, mk_eq(t, xsy, false));
         tightest_prefix(s, x, ~cnt);
     }
     else {
+        // 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));
+
         expr_ref x = mk_skolem(m_indexof_left, t, s, offset);
         expr_ref y = mk_skolem(m_indexof_right, t, s, offset);
         expr_ref indexof0(m_util.str.mk_index(y, s, zero), m);
@@ -2067,6 +2086,15 @@ literal theory_seq::mk_eq_empty(expr* _e) {
     expr_ref e(_e, m);
     SASSERT(m_util.is_seq(e));
     expr_ref emp(m);
+    zstring s;
+    if (m_util.str.is_string(e, s)) {
+        if (s.length() == 0) {
+            return true_literal;
+        }
+        else {
+            return false_literal;
+        }
+    }
     emp = m_util.str.mk_empty(m.get_sort(e));
     return mk_equals(e, emp);
 }
@@ -2074,10 +2102,11 @@ literal theory_seq::mk_eq_empty(expr* _e) {
 void theory_seq::add_axiom(literal l1, literal l2, literal l3, literal l4) {
     context& ctx = get_context();
     literal_vector lits;
-    if (l1 != null_literal) { ctx.mark_as_relevant(l1); lits.push_back(l1); }
-    if (l2 != null_literal) { ctx.mark_as_relevant(l2); lits.push_back(l2); }
-    if (l3 != null_literal) { ctx.mark_as_relevant(l3); lits.push_back(l3); }
-    if (l4 != null_literal) { ctx.mark_as_relevant(l4); lits.push_back(l4); }
+    if (l1 == true_literal || l2 == true_literal || l3 == true_literal || l4 == true_literal) return;
+    if (l1 != null_literal && l1 != false_literal) { ctx.mark_as_relevant(l1); lits.push_back(l1); }
+    if (l2 != null_literal && l2 != false_literal) { ctx.mark_as_relevant(l2); lits.push_back(l2); }
+    if (l3 != null_literal && l3 != false_literal) { ctx.mark_as_relevant(l3); lits.push_back(l3); }
+    if (l4 != null_literal && l4 != false_literal) { ctx.mark_as_relevant(l4); lits.push_back(l4); }
     TRACE("seq", ctx.display_literals_verbose(tout << "axiom: ", lits.size(), lits.c_ptr()); tout << "\n";);
     m_new_propagation = true;
     ++m_stats.m_add_axiom;
@@ -2117,8 +2146,8 @@ void theory_seq::propagate_eq(literal lit, expr* e1, expr* e2, bool add_to_eqs)
 
     }
     TRACE("seq",
-          tout << mk_pp(ctx.bool_var2expr(lit.var()), m) << " => "
-          << mk_pp(e1, m) << " = " << mk_pp(e2, m) << "\n";);
+          ctx.display_literals_verbose(tout, 1, &lit);
+          tout << " => " << mk_pp(e1, m) << " = " << mk_pp(e2, m) << "\n";);
     justification* js =
         ctx.mk_justification(
             ext_theory_eq_propagation_justification(
@@ -2161,7 +2190,7 @@ void theory_seq::assign_eh(bool_var v, bool is_true) {
             propagate_non_empty(lit, e1);
 
             // lit => e1 = first ++ (unit last)
-            expr_ref f1 = mk_skolem(m_seq_first, e1);
+            expr_ref f1 = mk_first(e1);
             expr_ref f2 = mk_last(e1);
             f = mk_concat(f1, m_util.str.mk_unit(f2));
             propagate_eq(lit, e1, f, true);
@@ -2271,6 +2300,7 @@ void theory_seq::push_scope_eh() {
 }
 
 void theory_seq::pop_scope_eh(unsigned num_scopes) {
+    context& ctx = get_context();
     m_trail_stack.pop_scope(num_scopes);
     theory::pop_scope_eh(num_scopes);
     m_dm.pop_scope(num_scopes);
@@ -2280,6 +2310,10 @@ void theory_seq::pop_scope_eh(unsigned num_scopes) {
     m_nqs.pop_scope(num_scopes);
     m_atoms.resize(m_atoms_lim[m_atoms_lim.size()-num_scopes]);
     m_atoms_lim.shrink(m_atoms_lim.size()-num_scopes);
+    m_rewrite.reset();    
+    if (ctx.get_base_level() > ctx.get_scope_level() - num_scopes) {
+        m_replay.reset();
+    }
 }
 
 void theory_seq::restart_eh() {
@@ -2606,7 +2640,8 @@ bool theory_seq::add_prefix2prefix(expr* e) {
         return false;
     }
     expr_ref emp(m_util.str.mk_empty(m.get_sort(e1)), m);
-    switch (assume_equality(e2, emp)) {
+    literal e2_is_emp = mk_eq_empty(e2);
+    switch (ctx.get_assignment(e2_is_emp)) {
     case l_true:
         return false; // done
     case l_undef:
@@ -2614,16 +2649,19 @@ bool theory_seq::add_prefix2prefix(expr* e) {
     default:
         break;
     }
-    expr_ref head1(m), tail1(m), head2(m), tail2(m);
+
+    expr_ref head1(m), tail1(m), head2(m), tail2(m), conc(m);
     mk_decompose(e1, head1, tail1);
     mk_decompose(e2, head2, tail2);
+    conc = mk_concat(head2, tail2);
+    propagate_eq(~e2_is_emp, e2, conc, true);
 
     literal lit = mk_eq(head1, head2, false);
     switch (ctx.get_assignment(lit)) {
     case l_true: {
         literal_vector lits;
         lits.push_back(~ctx.get_literal(e));
-        lits.push_back(~mk_eq(e2, emp, false));
+        lits.push_back(~e2_is_emp);
         lits.push_back(lit);
         propagate_lit(0, lits.size(), lits.c_ptr(), ~mk_literal(m_util.str.mk_prefix(tail1, tail2)));
         return false;
@@ -2650,28 +2688,32 @@ bool theory_seq::add_suffix2suffix(expr* e) {
     }
 
     expr_ref emp(m_util.str.mk_empty(m.get_sort(e1)), m);
-
-    switch (assume_equality(e2, emp)) {
+    literal e2_is_emp = mk_eq_empty(e2);
+    switch (ctx.get_assignment(e2_is_emp)) {
     case l_true:
+        TRACE("seq", tout << mk_pp(e, m) << " " << mk_pp(e2, m) << " is empty\n";);
         return false; // done
     case l_undef:
-        ctx.force_phase(mk_eq(e2, emp, false));
+        TRACE("seq", tout << mk_pp(e, m) << " " << mk_pp(e2, m) << " is unassigned\n";);
+        ctx.force_phase(e2_is_emp);
         return true;  // retry
     case l_false:
         break;
     }
-    expr_ref first2 = mk_skolem(m_seq_first, e2);
+    expr_ref first2 = mk_first(e2);
     expr_ref last2  = mk_last(e2);
-    expr_ref first1 = mk_skolem(m_seq_first, e1);
+    expr_ref first1 = mk_first(e1);
     expr_ref last1  = mk_last(e1);
     expr_ref conc = mk_concat(first2, m_util.str.mk_unit(last2));
-    propagate_eq(~mk_eq(e2, emp, false), e2, conc);
+    propagate_eq(~e2_is_emp, e2, conc, true);
 
     literal last_eq = mk_eq(last1, last2, false);
     switch (ctx.get_assignment(last_eq)) {
     case l_false:
+        TRACE("seq", tout << mk_pp(e, m) << " " << last1 << " = " << last2 << " is false\n";);
         return false; // done
     case l_undef:
+        TRACE("seq", tout << mk_pp(e, m) << " " << last1 << " = " << last2 << " is unassigned\n";);
         ctx.force_phase(~last_eq);
         return true;
     case l_true:
@@ -2680,9 +2722,10 @@ bool theory_seq::add_suffix2suffix(expr* e) {
 
     literal_vector lits;
     lits.push_back(~ctx.get_literal(e));
-    lits.push_back(~mk_eq(e2, emp, false));
+    lits.push_back(~e2_is_emp);
     lits.push_back(last_eq);
     propagate_lit(0, lits.size(), lits.c_ptr(), ~mk_literal(m_util.str.mk_suffix(first1, first2)));
+    TRACE("seq", tout << mk_pp(e, m) << " saturate\n";);
     return false;
 }
 
diff --git a/src/smt/theory_seq.h b/src/smt/theory_seq.h
index fd1219d4b..8ad9e7caf 100644
--- a/src/smt/theory_seq.h
+++ b/src/smt/theory_seq.h
@@ -192,6 +192,7 @@ namespace smt {
             expr_ref m_e;
         public:
             replay_length_coherence(ast_manager& m, expr* e) : m_e(e, m) {}
+            virtual ~replay_length_coherence() {}
             virtual void operator()(theory_seq& th) {
                 th.check_length_coherence(m_e);
                 m_e.reset();
@@ -202,6 +203,7 @@ namespace smt {
             expr_ref m_e;
         public:
             replay_axiom(ast_manager& m, expr* e) : m_e(e, m) {}
+            virtual ~replay_axiom() {}
             virtual void operator()(theory_seq& th) {
                 th.enque_axiom(m_e);
                 m_e.reset();
@@ -336,7 +338,7 @@ namespace smt {
         void propagate_lit(dependency* dep, literal lit) { propagate_lit(dep, 0, 0, lit); }
         void propagate_lit(dependency* dep, unsigned n, literal const* lits, literal lit);
         void propagate_eq(dependency* dep, enode* n1, enode* n2);
-        void propagate_eq(literal lit, expr* e1, expr* e2, bool add_to_eqs = false);
+        void propagate_eq(literal lit, expr* e1, expr* e2, bool add_to_eqs);
         void set_conflict(dependency* dep, literal_vector const& lits = literal_vector());
 
         u_map<unsigned> m_branch_start;
@@ -355,6 +357,7 @@ namespace smt {
         bool is_tail(expr* a, expr*& s, unsigned& idx) const;
         expr_ref mk_nth(expr* s, expr* idx);
         expr_ref mk_last(expr* e);
+        expr_ref mk_first(expr* e);
         expr_ref canonize(expr* e, dependency*& eqs);
         bool canonize(expr* e, expr_ref_vector& es, dependency*& eqs);
         bool canonize(expr_ref_vector const& es, expr_ref_vector& result, dependency*& eqs);