diff --git a/src/ast/sls/sls_arith_base.cpp b/src/ast/sls/sls_arith_base.cpp
index 0fb9b4d6e5a..590c4c6df6c 100644
--- a/src/ast/sls/sls_arith_base.cpp
+++ b/src/ast/sls/sls_arith_base.cpp
@@ -1968,6 +1968,7 @@ namespace sls {
 
     template<typename num_t>
     bool arith_base<num_t>::set_value(expr* e, expr* v) {
+
         if (!a.is_int_real(e))
             return false;
         var_t w = m_expr2var.get(e->get_id(), UINT_MAX);
@@ -1984,7 +1985,14 @@ namespace sls {
         }
         if (n == value(w))
             return true;
-         return update(w, n);        
+         bool r = update(w, n);   
+
+         if (!r) {
+             IF_VERBOSE(2,
+                 verbose_stream() << "set value failed " << mk_pp(e, m) << " := " << mk_pp(v, m) << "\n";
+                display(verbose_stream(), w) << " := " << value(w) << "\n");
+         }
+         return r;
     }
 
     template<typename num_t>
@@ -1996,6 +2004,23 @@ namespace sls {
         return expr_ref(a.mk_numeral(m_vars[v].value().to_rational(), a.is_int(e)), m);
     }
 
+    template<typename num_t>
+    bool arith_base<num_t>::is_fixed(expr* e, expr_ref& value) {
+        if (!a.is_int_real(e))
+            return false;
+        num_t n;
+        if (is_num(e, n)) {
+            value = expr_ref(a.mk_numeral(n.to_rational(), a.is_int(e)), m);
+            return true;
+        }
+        auto v = mk_term(e);
+        if (is_fixed(v)) {
+            value = expr_ref(a.mk_numeral(m_vars[v].value().to_rational(), a.is_int(e)), m);
+            return true;
+        }
+        return false;
+    }
+
     template<typename num_t>
     bool arith_base<num_t>::is_sat() {
         invariant();
diff --git a/src/ast/sls/sls_arith_base.h b/src/ast/sls/sls_arith_base.h
index 60a03cb36c6..df2f95ee4ca 100644
--- a/src/ast/sls/sls_arith_base.h
+++ b/src/ast/sls/sls_arith_base.h
@@ -280,6 +280,7 @@ namespace sls {
         void register_term(expr* e) override;
         bool set_value(expr* e, expr* v) override;
         expr_ref get_value(expr* e) override;
+        bool is_fixed(expr* e, expr_ref& value) override;
         void initialize() override;
         void propagate_literal(sat::literal lit) override;
         bool propagate() override;
diff --git a/src/ast/sls/sls_arith_plugin.cpp b/src/ast/sls/sls_arith_plugin.cpp
index d9275b4e73c..c8bbbfd5117 100644
--- a/src/ast/sls/sls_arith_plugin.cpp
+++ b/src/ast/sls/sls_arith_plugin.cpp
@@ -64,6 +64,10 @@ namespace sls {
         WITH_FALLBACK(get_value(e));
     }
 
+    bool arith_plugin::is_fixed(expr* e, expr_ref& value) {
+        WITH_FALLBACK(is_fixed(e, value));
+    }
+
     void arith_plugin::initialize() {
         APPLY_BOTH(initialize());
     }
diff --git a/src/ast/sls/sls_arith_plugin.h b/src/ast/sls/sls_arith_plugin.h
index 7d84915798f..7a04711107b 100644
--- a/src/ast/sls/sls_arith_plugin.h
+++ b/src/ast/sls/sls_arith_plugin.h
@@ -32,6 +32,7 @@ namespace sls {
         ~arith_plugin() override {}
         void register_term(expr* e) override;
         expr_ref get_value(expr* e) override;
+        bool is_fixed(expr* e, expr_ref& value) override;
         void initialize() override;
         void propagate_literal(sat::literal lit) override;
         bool propagate() override;
diff --git a/src/ast/sls/sls_context.cpp b/src/ast/sls/sls_context.cpp
index c55c2ac792a..b610e76edaa 100644
--- a/src/ast/sls/sls_context.cpp
+++ b/src/ast/sls/sls_context.cpp
@@ -172,6 +172,7 @@ namespace sls {
     }
     
     void context::propagate_boolean_assignment() {
+    start_propagation:
         reinit_relevant();
 
         for (auto p : m_plugins)
@@ -180,9 +181,12 @@ namespace sls {
 
         for (sat::literal lit : root_literals()) 
             propagate_literal(lit);
-
-        if (m_new_constraint)
-            return;        
+        
+        if (any_of(root_literals(), [&](sat::literal lit) { return !is_true(lit); })) {
+            if (unsat().empty())
+                goto start_propagation;
+            return;
+        }
 
         while (!m_new_constraint && m.inc() && (!m_repair_up.empty() || !m_repair_down.empty())) {
             while (!m_repair_down.empty() && !m_new_constraint && m.inc()) {
@@ -258,9 +262,6 @@ namespace sls {
         auto p = m_plugins.get(fid, nullptr);
         if (p)
             p->propagate_literal(lit);
-        if (!is_true(lit)) {
-            m_new_constraint = true;
-        }
     }
 
     bool context::is_true(expr* e) {
@@ -291,6 +292,14 @@ namespace sls {
     bool context::set_value(expr * e, expr * v) {
         return any_of(m_plugins, [&](auto p) { return p && p->set_value(e, v); });
     }
+
+    bool context::is_fixed(expr* e, expr_ref& value) {
+        if (m.is_value(e)) {
+            value = e;
+            return true;
+        }
+        return any_of(m_plugins, [&](auto p) { return p && p->is_fixed(e, value); });
+    }
     
     bool context::is_relevant(expr* e) {
         unsigned id = e->get_id();
@@ -317,6 +326,7 @@ namespace sls {
         m_constraint_trail.push_back(e);
         add_clause(e);     
         m_new_constraint = true;
+        verbose_stream() << "add constraint\n";
         ++m_stats.m_num_constraints;
     }
 
diff --git a/src/ast/sls/sls_context.h b/src/ast/sls/sls_context.h
index 95f9142922a..5ff1fd05250 100644
--- a/src/ast/sls/sls_context.h
+++ b/src/ast/sls/sls_context.h
@@ -42,6 +42,7 @@ namespace sls {
         virtual family_id fid() { return m_fid; }
         virtual void register_term(expr* e) = 0;
         virtual expr_ref get_value(expr* e) = 0;
+        virtual bool is_fixed(expr* e, expr_ref& value) { return false; }
         virtual void initialize() = 0;
         virtual void start_propagation() {};
         virtual bool propagate() = 0;
@@ -192,6 +193,7 @@ namespace sls {
 
         // Between plugin solvers
         expr_ref get_value(expr* e);
+        bool is_fixed(expr* v, expr_ref& value);
         bool set_value(expr* e, expr* v);
         void new_value_eh(expr* e);
         bool is_true(expr* e);
diff --git a/src/ast/sls/sls_seq_plugin.cpp b/src/ast/sls/sls_seq_plugin.cpp
index 9745ed74700..d8e42d234cf 100644
--- a/src/ast/sls/sls_seq_plugin.cpp
+++ b/src/ast/sls/sls_seq_plugin.cpp
@@ -63,7 +63,7 @@ Alternate to lookahead strategy:
   - create a priority buffer array of vector<ptr_vector<expr>> based on depth.
   - walk from lowest depth up. Reset each inner buffer when processed. Parents always 
     have higher depth.
-  - calculate repair/break score when hitting a predicate based on bval1.
+  - calculate repair/break depth when hitting a predicate based on bval1.
   - strval1 and bval1 are modified by 
     - use a global timestamp.
     - label each eval subterm by a timestamp that gets set.
@@ -145,9 +145,26 @@ namespace sls {
                 auto ve = ctx.get_value(e);
                 if (a.is_numeral(ve, r) && r == sx.length())
                     continue;
-                update(e, rational(sx.length()));
+                // set e to length of x or
+                // set x to a string of length e
+
+                if (r == 0 || sx.length() == 0) {
+                    verbose_stream() << "todo-create lemma: len(x) = 0 <=> x = \"\"\n";
+                    // create a lemma: len(x) = 0 => x = ""
+                }
+                if (ctx.rand(2) == 0 && update(e, rational(sx.length())))
+                    return false;
+                if (r < sx.length() && update(x, sx.extract(0, r.get_unsigned())))
+                    return false;
+                if (update(e, rational(sx.length())))
+                    return false;
+                if (r > sx.length() && update(x, sx + zstring(m_chars[ctx.rand(m_chars.size())])))
+                    return false;
+                verbose_stream() << mk_pp(x, m) << " = " << sx << " " << ve << "\n";
+                NOT_IMPLEMENTED_YET();
                 return false;
             }
+
             if ((seq.str.is_index(e, x, y, z) || seq.str.is_index(e, x, y)) && seq.is_string(x->get_sort())) {
                 auto sx = strval0(x);
                 auto sy = strval0(y);
@@ -603,21 +620,32 @@ namespace sls {
         auto const& R = rhs(eq);
         unsigned i = 0, j = 0;    // position into current string
         unsigned ni = 0, nj = 0;  // current string in concatenation
-        double depth = 1.0;       // priority of update. Doubled when depth of equal strings are increased.
+        double score = 1.0;       // priority of update. Doubled when score of equal strings are increased.
+
+        IF_VERBOSE(4,
+            verbose_stream() << "unify: \"" << strval0(eq->get_arg(0)) << "\" == \"" << strval0(eq->get_arg(1)) << "\"\n";
+            for (auto x : L) verbose_stream() << mk_pp(x, m) << " ";
+            verbose_stream() << " == ";
+            for (auto x : R) verbose_stream() << mk_pp(x, m) << " ";
+            verbose_stream() << "\n";
+            for (auto x : L) verbose_stream() << "\"" << strval0(x) << "\" ";
+            verbose_stream() << " == ";
+            for (auto x : R) verbose_stream() << "\"" << strval0(x) << "\" ";
+            verbose_stream() << "\n";
+            );
+
         while (ni < L.size() && nj < R.size()) {
             auto const& xi = L[ni];
             auto const& yj = R[nj];
             auto const& vi = strval0(xi);
             auto const& vj = strval0(yj);
-            IF_VERBOSE(4, 
-                verbose_stream() << "unify: \"" << vi << "\" = \"" << vj << "\" incides " << i << " " << j << "\n";
-                verbose_stream() << vi.length() << " " << vj.length() << "\n");
+
             if (vi.length() == i && vj.length() == j) {
-                depth *= 2;
+                score *= 2;
                 if (nj + 1 < R.size() && !strval0(R[nj + 1]).empty())
-                    m_str_updates.push_back({ xi, vi + zstring(strval0(R[nj + 1])[0]), depth });
+                    m_str_updates.push_back({ xi, vi + zstring(strval0(R[nj + 1])[0]), score });
                 if (ni + 1 < L.size() && !strval0(L[ni + 1]).empty())
-                    m_str_updates.push_back({ yj, vj + zstring(strval0(L[ni + 1])[0]), depth });
+                    m_str_updates.push_back({ yj, vj + zstring(strval0(L[ni + 1])[0]), score });
                 i = 0;
                 j = 0;
                 ++ni;
@@ -627,8 +655,8 @@ namespace sls {
             if (vi.length() == i) {
                 // xi -> vi + vj[j]
                 SASSERT(j < vj.length());
-                m_str_updates.push_back({ xi, vi + zstring(vj[j]), depth});
-                depth *= 2;
+                m_str_updates.push_back({ xi, vi + zstring(vj[j]), score});
+                score *= 2;
                 i = 0;
                 ++ni;
                 continue;
@@ -636,8 +664,8 @@ namespace sls {
             if (vj.length() == j) {
                 // yj -> vj + vi[i]
                 SASSERT(i < vi.length());
-                m_str_updates.push_back({ yj, vj + zstring(vi[i]), depth });
-                depth *= 2;
+                m_str_updates.push_back({ yj, vj + zstring(vi[i]), score });
+                score *= 2;
                 j = 0;
                 ++nj;
                 continue;
@@ -645,30 +673,33 @@ namespace sls {
             SASSERT(i < vi.length());
             SASSERT(j < vj.length());
             if (is_value(xi) && is_value(yj)) {
+                if (vi[i] != vj[j])
+                    score = 1;
                 ++i, ++j;
                 continue;
             }
+
             if (vi[i] == vj[j]) {
                 ++i, ++j;
                 continue;
             }
             if (!is_value(xi)) {
-                m_str_updates.push_back({ xi, vi.extract(0, i), depth });
-                m_str_updates.push_back({ xi, vi.extract(0, i) + zstring(vj[j]), depth});                
+                m_str_updates.push_back({ xi, vi.extract(0, i), score });
+                m_str_updates.push_back({ xi, vi.extract(0, i) + zstring(vj[j]), score});                
             }
             if (!is_value(yj)) {
-                m_str_updates.push_back({ yj, vj.extract(0, j), depth });
-                m_str_updates.push_back({ yj, vj.extract(0, j) + zstring(vi[i]), depth });
+                m_str_updates.push_back({ yj, vj.extract(0, j), score });
+                m_str_updates.push_back({ yj, vj.extract(0, j) + zstring(vi[i]), score });
             }
             break;
         }
         for (; ni < L.size(); ++ni) 
-            if (!is_value(L[ni]))
-                m_str_updates.push_back({ L[ni], zstring(), depth });
+            if (!is_value(L[ni]) && !strval0(L[ni]).empty())
+                m_str_updates.push_back({ L[ni], zstring(), 1 });
         
         for (; nj < R.size(); ++nj)
-            if (!is_value(R[nj]))
-                m_str_updates.push_back({ R[nj], zstring(), depth });
+            if (!is_value(R[nj]) && !strval0(R[nj]).empty())
+                m_str_updates.push_back({ R[nj], zstring(), 1 });
 
         return apply_update();
     }
@@ -1092,6 +1123,13 @@ namespace sls {
             sum_scores += score;
         for (auto const& [e, val, score] : m_int_updates)
             sum_scores += score;
+
+        IF_VERBOSE(4,
+            for (auto const& [e, val, score] : m_str_updates)
+                verbose_stream() << mk_pp(e, m) << " := \"" << val << "\" score: " << score << "\n";
+        for (auto const& [e, val, score] : m_int_updates)
+            verbose_stream() << mk_pp(e, m) << " := " << val << " score: " << score << "\n";
+            );
         
         while (!m_str_updates.empty() || !m_int_updates.empty()) {
             bool is_str_update = false;
@@ -1165,6 +1203,7 @@ namespace sls {
         if (m_initialized)
             return;
         m_initialized = true;
+        expr_ref val(m);
         for (auto lit : ctx.unit_literals()) {
             auto e = ctx.atom(lit.var());
             expr* x, * y, * z;
@@ -1214,6 +1253,12 @@ namespace sls {
                 if (len_r.is_unsigned())
                     ev.max_length = std::min(ev.max_length, len_r.get_unsigned());
             }
+            // TBD: assumes arithmetic is already initialized
+            if (seq.str.is_length(t, x) && ctx.is_fixed(t, val) && 
+                a.is_numeral(val, len_r) && len_r.is_unsigned()) {
+                auto& ev = get_eval(x);
+                ev.min_length = ev.max_length = len_r.get_unsigned();                
+            }
         }
     }
     
diff --git a/src/tactic/sls/sls_tactic.cpp b/src/tactic/sls/sls_tactic.cpp
index e72c01af0d7..f50edcaccd5 100644
--- a/src/tactic/sls/sls_tactic.cpp
+++ b/src/tactic/sls/sls_tactic.cpp
@@ -78,7 +78,8 @@ class sls_smt_tactic : public tactic {
         try {
             res = m_sls->check();
         }
-        catch (z3_exception&) {
+        catch (z3_exception& ex) {
+            IF_VERBOSE(1, verbose_stream() << "SLS threw an exception: " << ex.what() << "\n");
             m_sls->collect_statistics(m_st);
             throw;
         }
@@ -98,7 +99,6 @@ class sls_smt_tactic : public tactic {
         }
         else
             mc = nullptr;
-
     }
 
     void operator()(goal_ref const& g,