diff --git a/examples/python/rc2.py b/examples/python/rc2.py
index a68e41b46..5ccbbaca8 100644
--- a/examples/python/rc2.py
+++ b/examples/python/rc2.py
@@ -1,5 +1,9 @@
 # RC2 algorithm
-# basic version without optimizations
+# basic version with some optimizations
+# - process soft constraints in order of highest values first.
+# - extract multiple cores, not just one
+# - use built-in cardinality constraints, cheap core minimization. 
+#
 # See also https://github.com/pysathq/pysat and papers in CP 2014, JSAT 2015.
 
 from z3 import *
@@ -7,7 +11,6 @@ from z3 import *
 def tt(s, f):
     return is_true(s.model().eval(f))
 
-
 def add(Ws, f, w):
     Ws[f] = w + (Ws[f] if f in Ws else 0)
 
@@ -19,46 +22,106 @@ def sub(Ws, f, w):
         del(Ws[f])
 
 class RC2:
+
     def __init__(self, s):
         self.bounds = {}
+        self.names = {}
         self.solver = s
         self.solver.set("sat.cardinality.solver", True)
         self.solver.set("sat.core.minimize", True)
         self.solver.set("sat.core.minimize_partial", True)
         
     def at_most(self, S, k):
-        fml = AtMost(S + [k])
+        fml = simplify(AtMost(S + [k]))
+        if fml in self.names:
+           return self.names[fml]
         name = Bool("%s" % fml)
         self.solver.add(Implies(name, fml))
         self.bounds[name] = (S, k)
+        sel.names[fml] = name
         return name
 
+    def print_cost(self):
+        print("max cost", self.max_cost, "min cost", self.min_cost)
+
+    def update_max_cost(self):
+        self.max_cost = min(self.max_cost, self.get_cost())
+        self.print_cost()
+    
+    # sort W, and incrementally add elements of W
+    # in sorted order to prefer cores with high weight.
+    def check(self, Ws):
+        ws = sorted(list(Ws), lambda f,w : -w)
+        # print(ws)
+        i = 0
+        while i < len(ws):
+           j = i
+           # increment j until making 5% progress or exhausting equal weight entries
+           while (j < len(ws) and ws[j][1] == ws[i][1]) or (i > 0 and (i - j)*20 < len(ws)):
+              j += 1
+           i = j
+           r = self.solver.check(ws[j][0] for j in range(i))
+           if r == sat:
+              self.update_max_cost()
+           else:
+              return r
+        return sat
+             
+    def get_cost(self):
+        return sum(self.Ws0[c] for c in self.Ws0 if not tt(self.solver, c))
+
+    # Retrieve independendent cores from Ws
+    def get_cores(self, Ws):
+        cores = []
+        while unsat == self.check(Ws):            
+            core = list(self.solver.unsat_core())
+            print (self.solver.statistics())
+            if not core:
+               return unsat
+            w = min([Ws[c] for c in core])
+            for f in core:
+                sub(Ws, f, w)
+            cores += [(core, w)]
+        self.update_max_cost()
+        return cores
+           
+    # Add new soft constraints to replace core
+    # with weight w. Allow to weaken at most
+    # one element of core. Elements that are
+    # cardinality constraints are weakened by
+    # increasing their bounds. Non-cardinality
+    # constraints are weakened to "true". They
+    # correspond to the constraint Not(s) <= 0, 
+    # so weakening produces Not(s) <= 1, which
+    # is a tautology.
+    def update_bounds(self, Ws, core, w):
+        for f in core:
+           if f in self.bounds:
+              S, k = self.bounds[f]
+              if k + 1 < len(S):
+                 add(Ws, self.at_most(S, k + 1), w)                
+        add(Ws, self.at_most([mk_not(f) for f in core], 1), w)
+
     # Ws are weighted soft constraints
     # Whenever there is an unsatisfiable core over ws
     # increase the limit of each soft constraint from a bound
     # and create a soft constraint that limits the number of
     # increased bounds to be at most one.
     def maxsat(self, Ws):
-        cost = 0
-        Ws0 = Ws.copy()
-        while unsat == self.solver.check([f for f in Ws]):
-            core = list(self.solver.unsat_core())
-            print (self.solver.statistics())
-            print("Core", core)
-            if not core:
-                return unsat
-            w = min([Ws[c] for c in core])
-            cost += w
-            for f in core:
-                sub(Ws, f, w)
-            for f in core:
-                if f in self.bounds:
-                    S, k = self.bounds[f]
-                    if k + 1 < len(S):
-                        add(Ws, self.at_most(S, k + 1), w)                
-            add(Ws, self.at_most([mk_not(f) for f in core], 1), w)
-            
-        return cost, { f for f in Ws0 if tt(self.solver, f) }
+        self.min_cost = 0
+        self.max_cost = sum(Ws[c] for c in Ws)
+        self.Ws0 = Ws.copy()
+        while True:
+            cores = self.get_cores(Ws)
+            if not cores:
+                break
+            if cores == unsat:
+               return unsat
+            for (core, w) in cores:
+               self.min_cost += w
+               self.print_cost()
+               self.update_bounds(Ws, core, w)            
+        return sel.min_cost, { f for f in self.Ws0 if not tt(self.solver, f) }
 
     def from_file(self, file):
         opt = Optimize()
@@ -71,12 +134,11 @@ class RC2:
             add(Ws, f.arg(0), f.arg(2).as_long())
         return self.maxsat(Ws)
 
-
 def main(file):
     s = SolverFor("QF_FD")
     rc2 = RC2(s)
     set_param(verbose=0)
-    cost, trues = rc2.from_file(file)
+    cost, falses = rc2.from_file(file)
     print(cost)
     print(s.statistics())
 
diff --git a/src/ast/rewriter/seq_rewriter.cpp b/src/ast/rewriter/seq_rewriter.cpp
index 75b0c38e9..6503bdcb9 100644
--- a/src/ast/rewriter/seq_rewriter.cpp
+++ b/src/ast/rewriter/seq_rewriter.cpp
@@ -675,25 +675,17 @@ br_status seq_rewriter::mk_seq_contains(expr* a, expr* b, expr_ref& result) {
         result = m().mk_bool_val(c.contains(d));
         return BR_DONE;
     }
+    expr* x = nullptr, *y, *z;
+    if (m_util.str.is_extract(b, x, y, z) && x == a) {
+        result = m().mk_true();
+        return BR_DONE;
+    }
+
     // check if subsequence of a is in b.
     expr_ref_vector as(m()), bs(m());
-    if (m_util.str.is_string(a, c)) {
-        for (unsigned i = 0; i < c.length(); ++i) {
-            as.push_back(m_util.str.mk_unit(m_util.str.mk_char(c, i)));
-        }
-    }
-    else {
-        m_util.str.get_concat(a, as);
-    }
-    if (m_util.str.is_string(b, d)) {
-        for (unsigned i = 0; i < d.length(); ++i) {
-            bs.push_back(m_util.str.mk_unit(m_util.str.mk_char(d, i)));
-        }
-    }
-    else {
-        m_util.str.get_concat(b, bs);
-    }
-    bool all_values = true;
+    m_util.str.get_concat_units(a, as);
+    m_util.str.get_concat_units(b, bs);
+    
     TRACE("seq", tout << mk_pp(a, m()) << " contains " << mk_pp(b, m()) << "\n";);
    
     if (bs.empty()) {
@@ -701,24 +693,21 @@ br_status seq_rewriter::mk_seq_contains(expr* a, expr* b, expr_ref& result) {
         return BR_DONE;
     }
 
-    for (unsigned i = 0; all_values && i < bs.size(); ++i) { 
-        all_values = m().is_value(bs[i].get());
+    if (as.empty()) {
+        result = m().mk_eq(b, m_util.str.mk_empty(m().get_sort(b)));
+        return BR_REWRITE2;
     }
 
-    bool found = false;
-    for (unsigned i = 0; !found && i < as.size(); ++i) {
-        all_values &= m().is_value(as[i].get());
-        if (bs.size() <= as.size() - i) {
-            unsigned j = 0;
-            for (; j < bs.size() && as[j+i].get() == bs[j].get(); ++j) {};
-            found = j == bs.size();
+    for (unsigned i = 0; bs.size() + i <= as.size(); ++i) {
+        unsigned j = 0;
+        for (; j < bs.size() && as.get(j+i) == bs.get(j); ++j) {};
+        if (j == bs.size()) {
+            result = m().mk_true();
+            return BR_DONE;
         }
     }
-    if (found) {
-        result = m().mk_true();
-        return BR_DONE;
-    }
-    if (all_values) {
+    std::function<bool(expr*)> is_value = [&](expr* e) { return m().is_value(e); };
+    if (bs.forall(is_value) && as.forall(is_value)) {
         result = m().mk_false();
         return BR_DONE;
     }
@@ -733,26 +722,12 @@ br_status seq_rewriter::mk_seq_contains(expr* a, expr* b, expr_ref& result) {
         }
     }
 
-    if (as.empty()) {
-        result = m().mk_eq(b, m_util.str.mk_empty(m().get_sort(b)));
-        return BR_REWRITE2;
-    }
-
-    if (bs.size() == 1 && m_util.str.is_string(bs[0].get(), c)) {
-        for (auto a_i : as) {
-            if (m_util.str.is_string(a_i, d) && d.contains(c)) {
-                result = m().mk_true();
-                return BR_DONE;
-            }
-        }
-    }
-
     unsigned offs = 0;
     unsigned sz = as.size();
-    expr* b0 = bs[0].get();
-    expr* bL = bs[bs.size()-1].get();
+    expr* b0 = bs.get(0);
+    expr* bL = bs.get(bs.size()-1);
     for (; offs < as.size() && cannot_contain_prefix(as[offs].get(), b0); ++offs) {}
-    for (; sz > offs && cannot_contain_suffix(as[sz-1].get(), bL); --sz) {}
+    for (; sz > offs && cannot_contain_suffix(as.get(sz-1), bL); --sz) {}
     if (offs == sz) {
         result = m().mk_eq(b, m_util.str.mk_empty(m().get_sort(b)));
         return BR_REWRITE2;
@@ -763,24 +738,15 @@ br_status seq_rewriter::mk_seq_contains(expr* a, expr* b, expr_ref& result) {
         return BR_REWRITE2;
     }    
 
-    expr* x, *y, *z;
-    if (m_util.str.is_extract(b, x, y, z) && x == a) {
-        result = m().mk_true();
-        return BR_DONE;
-    }
 
     std::function<bool(expr*)> is_unit = [&](expr *e) { return m_util.str.is_unit(e); };
 
     if (bs.forall(is_unit) && as.forall(is_unit)) {
-        if (as.size() < bs.size()) {
-            result = m().mk_false();
-            return BR_DONE;
-        }
         expr_ref_vector ors(m());
-        for (unsigned i = 0; i < as.size() - bs.size() + 1; ++i) {
+        for (unsigned i = 0; i + bs.size() <= as.size(); ++i) {
             expr_ref_vector ands(m());
             for (unsigned j = 0; j < bs.size(); ++j) {
-                ands.push_back(m().mk_eq(as[i + j].get(), bs[j].get()));
+                ands.push_back(m().mk_eq(as.get(i + j), bs.get(j)));
             }
             ors.push_back(::mk_and(ands));
         }
@@ -797,46 +763,41 @@ 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) {
     zstring c;
     rational r;
-    if (m_autil.is_numeral(b, r)) {
-        if (r.is_neg()) {
-            result = m_util.str.mk_empty(m().get_sort(a));
-            return BR_DONE;
-        } 
-        unsigned len = 0;
-        bool bounded = min_length(1, &a, len);
-        if (bounded && r >= rational(len)) {
-            result = m_util.str.mk_empty(m().get_sort(a));
-            return BR_DONE;
-        }
-        if (m_util.str.is_string(a, c)) {
-            if (r.is_unsigned() && r < rational(c.length())) {
-                result = m_util.str.mk_string(c.extract(r.get_unsigned(), 1));
-            }
-            else {
-                result = m_util.str.mk_empty(m().get_sort(a));
-            }
-            return BR_DONE;
-        }
-        if (r.is_unsigned()) {
-            len = r.get_unsigned();
-            expr_ref_vector as(m());
-            m_util.str.get_concat(a, as);
-            for (unsigned i = 0; i < as.size(); ++i) {
-                if (m_util.str.is_unit(as[i].get())) {
-                    if (len == 0) {
-                        result = as[i].get();
-                        return BR_DONE;
-                    }
-                    --len;
-                }         
-                else {
-                    return BR_FAILED;
-                }
-            }
-        }
-        
+    if (!m_autil.is_numeral(b, r)) {
+        return BR_FAILED;
     }
-    return BR_FAILED;
+    if (r.is_neg()) {
+        result = m_util.str.mk_empty(m().get_sort(a));
+        return BR_DONE;
+    } 
+    if (!r.is_unsigned()) {
+        return BR_FAILED;
+    }
+    unsigned len = r.get_unsigned();
+
+    expr_ref_vector as(m());
+    m_util.str.get_concat_units(a, as);
+
+    for (unsigned i = 0; i < as.size(); ++i) {
+        expr* a = as.get(i);
+        if (m_util.str.is_unit(a)) {
+            if (len == i) {
+                result = a;
+                return BR_REWRITE1;
+            }            
+        }
+        else if (i > 0) {
+            SASSERT(len >= i);
+            result = m_util.str.mk_concat(as.size() - i, as.c_ptr() + i);
+            result = m().mk_app(m_util.get_family_id(), OP_SEQ_AT, result, m_autil.mk_int(len - i));
+            return BR_REWRITE2;
+        }
+        else {
+            return BR_FAILED;
+        }
+    }
+    result = m_util.str.mk_empty(m().get_sort(a));
+    return BR_DONE;
 }
 
 br_status seq_rewriter::mk_seq_index(expr* a, expr* b, expr* c, expr_ref& result) {
diff --git a/src/ast/seq_decl_plugin.cpp b/src/ast/seq_decl_plugin.cpp
index 737213477..5a01b63bb 100644
--- a/src/ast/seq_decl_plugin.cpp
+++ b/src/ast/seq_decl_plugin.cpp
@@ -933,16 +933,16 @@ 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); }
+app* seq_util::str::mk_string(zstring const& s) const { 
+    return u.seq.mk_string(s); 
+}
 
-
-
-app*  seq_util::str::mk_char(zstring const& s, unsigned idx) {
+app*  seq_util::str::mk_char(zstring const& s, unsigned idx) const {
     bv_util bvu(m);
     return bvu.mk_numeral(s[idx], s.num_bits());
 }
 
-app*  seq_util::str::mk_char(char ch) {
+app*  seq_util::str::mk_char(char ch) const {
     zstring s(ch, zstring::ascii);
     return mk_char(s, 0);
 }
@@ -969,6 +969,24 @@ void seq_util::str::get_concat(expr* e, expr_ref_vector& es) const {
     }
 }
 
+void seq_util::str::get_concat_units(expr* e, expr_ref_vector& es) const {
+    expr* e1, *e2;
+    while (is_concat(e, e1, e2)) {
+        get_concat_units(e1, es);
+        e = e2;
+    }
+    zstring s;
+    if (is_string(e, s)) {
+        unsigned sz = s.length();
+        for (unsigned j = 0; j < sz; ++j) {
+            es.push_back(mk_unit(mk_char(s, j)));
+        }        
+    }
+    else if (!is_empty(e)) {
+        es.push_back(e);
+    }
+}
+
 app* seq_util::re::mk_loop(expr* r, unsigned lo) {
     parameter param(lo);
     return m.mk_app(m_fid, OP_RE_LOOP, 1, &param, 1, &r);
diff --git a/src/ast/seq_decl_plugin.h b/src/ast/seq_decl_plugin.h
index 8bd4d2807..80b432f3e 100644
--- a/src/ast/seq_decl_plugin.h
+++ b/src/ast/seq_decl_plugin.h
@@ -232,26 +232,26 @@ public:
     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); }
+        sort* mk_seq(sort* s) const { parameter param(s); return m.mk_sort(m_fid, SEQ_SORT, 1, &param); }
         sort* mk_string_sort() const { return m.mk_sort(m_fid, _STRING_SORT, 0, nullptr); }
         app* mk_empty(sort* s) const { return m.mk_const(m.mk_func_decl(m_fid, OP_SEQ_EMPTY, 0, nullptr, 0, (expr*const*)nullptr, s)); }
-        app* mk_string(zstring const& s);
-        app* mk_string(symbol const& s) { return u.seq.mk_string(s); }
-        app* mk_char(char ch);
+        app* mk_string(zstring const& s) const;
+        app* mk_string(symbol const& s) const { return u.seq.mk_string(s); }
+        app* mk_char(char ch) const;
         app* mk_concat(expr* a, expr* b) const { 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(a, mk_concat(b, c)); }
+        app* mk_concat(expr* a, expr* b, expr* c) const { return mk_concat(a, mk_concat(b, c)); }
         expr* mk_concat(unsigned n, expr* const* es) const { if (n == 1) return es[0]; SASSERT(n > 1); return m.mk_app(m_fid, OP_SEQ_CONCAT, n, es); }
         expr* mk_concat(expr_ref_vector const& es) const { return mk_concat(es.size(), es.c_ptr()); }
         app* mk_length(expr* a) const { return m.mk_app(m_fid, OP_SEQ_LENGTH, 1, &a); }
-        app* mk_substr(expr* a, expr* b, expr* c) { expr* es[3] = { a, b, c }; return m.mk_app(m_fid, OP_SEQ_EXTRACT, 3, es); }
-        app* mk_contains(expr* a, expr* b) { expr* es[2] = { a, b }; return m.mk_app(m_fid, OP_SEQ_CONTAINS, 2, es); }
-        app* mk_prefix(expr* a, expr* b) { expr* es[2] = { a, b }; return m.mk_app(m_fid, OP_SEQ_PREFIX, 2, es); }
-        app* mk_suffix(expr* a, expr* b) { expr* es[2] = { a, b }; return m.mk_app(m_fid, OP_SEQ_SUFFIX, 2, es); }
-        app* mk_index(expr* a, expr* b, expr* i) { expr* es[3] = { a, b, i}; return m.mk_app(m_fid, OP_SEQ_INDEX, 3, es); }
-        app* mk_unit(expr* u) { return m.mk_app(m_fid, OP_SEQ_UNIT, 1, &u); }
-        app* mk_char(zstring const& s, unsigned idx);
-        app* mk_itos(expr* i) { return m.mk_app(m_fid, OP_STRING_ITOS, 1, &i); }
-        app* mk_stoi(expr* s) { return m.mk_app(m_fid, OP_STRING_STOI, 1, &s); }
+        app* mk_substr(expr* a, expr* b, expr* c) const { expr* es[3] = { a, b, c }; return m.mk_app(m_fid, OP_SEQ_EXTRACT, 3, es); }
+        app* mk_contains(expr* a, expr* b) const { expr* es[2] = { a, b }; return m.mk_app(m_fid, OP_SEQ_CONTAINS, 2, es); }
+        app* mk_prefix(expr* a, expr* b) const { expr* es[2] = { a, b }; return m.mk_app(m_fid, OP_SEQ_PREFIX, 2, es); }
+        app* mk_suffix(expr* a, expr* b) const { expr* es[2] = { a, b }; return m.mk_app(m_fid, OP_SEQ_SUFFIX, 2, es); }
+        app* mk_index(expr* a, expr* b, expr* i) const { expr* es[3] = { a, b, i}; return m.mk_app(m_fid, OP_SEQ_INDEX, 3, es); }
+        app* mk_unit(expr* u) const { return m.mk_app(m_fid, OP_SEQ_UNIT, 1, &u); }
+        app* mk_char(zstring const& s, unsigned idx) const;
+        app* mk_itos(expr* i) const { return m.mk_app(m_fid, OP_STRING_ITOS, 1, &i); }
+        app* mk_stoi(expr* s) const { return m.mk_app(m_fid, OP_STRING_STOI, 1, &s); }
 
         bool is_string(expr const * n) const { return is_app_of(n, m_fid, OP_STRING_CONST); }
 
@@ -304,6 +304,7 @@ public:
         MATCH_UNARY(is_unit);
 
         void get_concat(expr* e, expr_ref_vector& es) const;
+        void get_concat_units(expr* e, expr_ref_vector& es) const;
         expr* get_leftmost_concat(expr* e) const { expr* e1, *e2; while (is_concat(e, e1, e2)) e = e1; return e; }
         expr* get_rightmost_concat(expr* e) const { expr* e1, *e2; while (is_concat(e, e1, e2)) e = e2; return e; }
     };