diff --git a/src/cmd_context/check_logic.cpp b/src/cmd_context/check_logic.cpp
index 191a3ccc1..733689ac9 100644
--- a/src/cmd_context/check_logic.cpp
+++ b/src/cmd_context/check_logic.cpp
@@ -20,6 +20,7 @@ Revision History:
 #include"arith_decl_plugin.h"
 #include"array_decl_plugin.h"
 #include"bv_decl_plugin.h"
+#include"seq_decl_plugin.h"
 #include"ast_pp.h"
 #include"for_each_expr.h"
 
@@ -29,6 +30,7 @@ struct check_logic::imp {
     arith_util    m_a_util;
     bv_util       m_bv_util;
     array_util    m_ar_util;
+    seq_util      m_seq_util;
     bool          m_uf;        // true if the logic supports uninterpreted functions
     bool          m_arrays;    // true if the logic supports arbitrary arrays
     bool          m_bv_arrays; // true if the logic supports only bv arrays
@@ -40,7 +42,7 @@ struct check_logic::imp {
     bool          m_quantifiers; // true if the logic supports quantifiers
     bool          m_unknown_logic;
 
-    imp(ast_manager & _m):m(_m), m_a_util(m), m_bv_util(m), m_ar_util(m) {
+    imp(ast_manager & _m):m(_m), m_a_util(m), m_bv_util(m), m_ar_util(m), m_seq_util(m) {
         reset();
     }
 
@@ -168,9 +170,12 @@ struct check_logic::imp {
             m_bvs         = true;
             m_quantifiers = true;
         }
-        else if (logic == "UF_S") {
+        else if (logic == "QF_S") {
             m_uf          = true;
             m_bvs         = true;
+            m_ints        = true;
+            m_arrays      = true;
+            m_reals       = true;
             m_quantifiers = false;
         }
         else {
@@ -424,6 +429,9 @@ struct check_logic::imp {
         else if (m.is_builtin_family_id(fid)) {
             // nothing to check
         }
+        else if (fid == m_seq_util.get_family_id()) {
+            // nothing to check
+        }
         else {
             fail("logic does not support theory");
         }
diff --git a/src/cmd_context/cmd_context.cpp b/src/cmd_context/cmd_context.cpp
index 0dbba83d6..3e76481d6 100644
--- a/src/cmd_context/cmd_context.cpp
+++ b/src/cmd_context/cmd_context.cpp
@@ -544,6 +544,7 @@ bool cmd_context::logic_has_arith_core(symbol const & s) const {
         s == "QF_FP" ||
         s == "QF_FPBV" ||
         s == "QF_BVFP" ||
+        s == "QF_S" ||
         s == "HORN";
 }
 
diff --git a/src/smt/smt_context.cpp b/src/smt/smt_context.cpp
index 00971f794..0be27bc08 100644
--- a/src/smt/smt_context.cpp
+++ b/src/smt/smt_context.cpp
@@ -312,6 +312,8 @@ namespace smt {
         d.m_phase_available        = true;
         d.m_phase                  = !l.sign();
         TRACE("phase_selection", tout << "saving phase, is_pos: " << d.m_phase << " l: " << l << "\n";);
+        TRACE("relevancy", 
+              tout << "is_atom: " << d.is_atom() << " is relevant: " << is_relevant_core(bool_var2expr(l.var())) << "\n";);
         if (d.is_atom() && (m_fparams.m_relevancy_lvl == 0 || (m_fparams.m_relevancy_lvl == 1 && !d.is_quantifier()) || is_relevant_core(bool_var2expr(l.var()))))
             m_atom_propagation_queue.push_back(l);
 
@@ -805,8 +807,10 @@ namespace smt {
     void context::merge_theory_vars(enode * n2, enode * n1, eq_justification js) {
         enode * r2 = n2->get_root();
         enode * r1 = n1->get_root();
-        if (!r1->has_th_vars() && !r2->has_th_vars())
+        if (!r1->has_th_vars() && !r2->has_th_vars()) {
+            TRACE("merge_theory_vars", tout << "Neither have theory vars #" << n1->get_owner()->get_id() << " #" << n2->get_owner()->get_id() << "\n";);
             return;
+        }
         
         theory_id from_th = null_theory_id;
 
diff --git a/src/smt/theory_seq.cpp b/src/smt/theory_seq.cpp
index 18b223eee..a402437d3 100644
--- a/src/smt/theory_seq.cpp
+++ b/src/smt/theory_seq.cpp
@@ -469,6 +469,9 @@ bool theory_seq::simplify_and_solve_eqs() {
     return change;
 }
 
+void theory_seq::internalize_eq_eh(app * atom, bool_var v) {
+}
+
 bool theory_seq::internalize_atom(app* a, bool) { 
     return internalize_term(a);
 }
@@ -598,7 +601,7 @@ void theory_seq::set_incomplete(app* term) {
 }
 
 theory_var theory_seq::mk_var(enode* n) {
-    if (!m_util.is_seq(n->get_owner()) ||
+    if (!m_util.is_seq(n->get_owner()) &&
         !m_util.is_re(n->get_owner())) {
         return null_theory_var;
     }
@@ -608,6 +611,7 @@ theory_var theory_seq::mk_var(enode* n) {
     else {
         theory_var v = theory::mk_var(n);
         get_context().attach_th_var(n, this, v);
+        get_context().mark_as_relevant(n);
         return v;
     }
 }
@@ -1005,6 +1009,10 @@ void theory_seq::assign_eq(bool_var v, bool is_true) {
         else if (m_util.str.is_in_re(e, e1, e2)) {
             // TBD
         }
+        else if (m.is_eq(e, e1, e2)) {
+            new_eq_eh(ctx.get_enode(e1)->get_th_var(get_id()),
+                      ctx.get_enode(e1)->get_th_var(get_id()));
+        }
         else {
             UNREACHABLE();
         }
diff --git a/src/smt/theory_seq.h b/src/smt/theory_seq.h
index f1010f73c..28a35c564 100644
--- a/src/smt/theory_seq.h
+++ b/src/smt/theory_seq.h
@@ -128,6 +128,7 @@ namespace smt {
         virtual final_check_status final_check_eh();
         virtual bool internalize_atom(app*, bool);
         virtual bool internalize_term(app*);
+        virtual void internalize_eq_eh(app * atom, bool_var v);
         virtual void new_eq_eh(theory_var, theory_var);
         virtual void new_diseq_eh(theory_var, theory_var);
         virtual void assign_eq(bool_var v, bool is_true);