diff --git a/src/smt/CMakeLists.txt b/src/smt/CMakeLists.txt
index f7a17a0ef..ba1c58cdb 100644
--- a/src/smt/CMakeLists.txt
+++ b/src/smt/CMakeLists.txt
@@ -80,6 +80,7 @@ z3_add_component(smt
     theory_str_regex.cpp
     theory_utvpi.cpp
     theory_wmaxsat.cpp
+    user_propagator.cpp
     uses_theory.cpp
     watch_list.cpp
   COMPONENT_DEPENDENCIES
diff --git a/src/smt/smt_context.cpp b/src/smt/smt_context.cpp
index e4dc193ca..b869be69e 100644
--- a/src/smt/smt_context.cpp
+++ b/src/smt/smt_context.cpp
@@ -853,7 +853,7 @@ namespace smt {
                     assign(l, mk_justification(eq_root_propagation_justification(curr)));
                 curr = curr->m_next;
             }
-            while(curr != r1);
+            while (curr != r1);
         }
         else {
             bool_var v1 = enode2bool_var(n1);
@@ -1394,6 +1394,8 @@ namespace smt {
         bool sign  = val == l_false;
         if (n->merge_tf())
             add_eq(n, sign ? m_false_enode : m_true_enode, eq_justification(literal(v, sign)));
+        if (watches_fixed(n)) 
+            assign_fixed(n, sign ? m.mk_false() : m.mk_true(), literal(v, sign));
         enode * r = n->get_root();
         if (r == m_true_enode || r == m_false_enode)
             return;
@@ -1924,6 +1926,8 @@ namespace smt {
 
         for (theory* t : m_theory_set) 
             t->push_scope_eh();
+        if (m_user_propagator) 
+            m_user_propagator->push_scope_eh();
         CASSERT("context", check_invariant());
     }
 
@@ -2418,9 +2422,11 @@ namespace smt {
             unassign_vars(s.m_assigned_literals_lim);
             undo_trail_stack(s.m_trail_stack_lim);
 
-            for (theory* th : m_theory_set) {
+            for (theory* th : m_theory_set) 
                 th->pop_scope_eh(num_scopes);
-            }
+
+            if (m_user_propagator) 
+                m_user_propagator->pop_scope_eh(num_scopes);
 
             del_justifications(m_justifications, s.m_justifications_lim);
 
@@ -2872,6 +2878,26 @@ namespace smt {
         }
     }
 
+    void context::register_user_propagator(
+        void* ctx, 
+        std::function<void(void*, unsigned, expr*)>& fixed_eh,
+        std::function<void(void*)>&                  push_eh,
+        std::function<void(void*, unsigned)>&        pop_eh) {
+        m_user_propagator = alloc(user_propagator, *this);
+        m_user_propagator->add(ctx, fixed_eh, push_eh, pop_eh);
+        for (unsigned i = m_scopes.size(); i-- > 0; ) 
+            m_user_propagator->push_scope_eh();
+    }
+
+    bool context::watches_fixed(enode* n) const {
+        return m_user_propagator && n->get_th_var(m_user_propagator->get_family_id()) != null_theory_var;
+    }
+
+    void context::assign_fixed(enode* n, expr* val, unsigned sz, literal const* explain) {
+        theory_var v = n->get_th_var(m_user_propagator->get_family_id());
+        m_user_propagator->new_fixed_eh(v, val, sz, explain);
+    }
+
     void context::push() {       
         pop_to_base_lvl();
         setup_context(false);
diff --git a/src/smt/smt_context.h b/src/smt/smt_context.h
index 0fed7e459..e425e025b 100644
--- a/src/smt/smt_context.h
+++ b/src/smt/smt_context.h
@@ -43,12 +43,13 @@ Revision History:
 #include "ast/ast_smt_pp.h"
 #include "smt/watch_list.h"
 #include "util/trail.h"
-#include "smt/fingerprints.h"
 #include "util/ref.h"
-#include "smt/proto_model/proto_model.h"
-#include "model/model.h"
 #include "util/timer.h"
 #include "util/statistics.h"
+#include "smt/fingerprints.h"
+#include "smt/proto_model/proto_model.h"
+#include "smt/user_propagator.h"
+#include "model/model.h"
 #include "solver/progress_callback.h"
 #include <tuple>
 
@@ -92,6 +93,7 @@ namespace smt {
         scoped_ptr<quantifier_manager>   m_qmanager;
         scoped_ptr<model_generator>      m_model_generator;
         scoped_ptr<relevancy_propagator> m_relevancy_propagator;
+        scoped_ptr<user_propagator>      m_user_propagator;
         random_gen                  m_random;
         bool                        m_flushing; // (debug support) true when flushing
         mutable unsigned            m_lemma_id;
@@ -1677,6 +1679,27 @@ namespace smt {
 
         void get_assertions(ptr_vector<expr> & result) { m_asserted_formulas.get_assertions(result); }
 
+        /*
+         * user-propagator
+         */
+        void register_user_propagator(
+            void* ctx, 
+            std::function<void(void*, unsigned, expr*)>& fixed_eh,
+            std::function<void(void*)>&                  push_eh,
+            std::function<void(void*, unsigned)>&        pop_eh);
+
+        bool watches_fixed(enode* n) const;
+
+        void assign_fixed(enode* n, expr* val, unsigned sz, literal const* explain);
+
+        void assign_fixed(enode* n, expr* val, literal_vector const& explain) {
+            assign_fixed(n, val, explain.size(), explain.c_ptr());
+        }
+
+        void assign_fixed(enode* n, expr* val, literal explain) {
+            assign_fixed(n, val, 1, &explain);
+        }
+
         void display(std::ostream & out) const;
 
         void display_unsat_core(std::ostream & out) const;
diff --git a/src/smt/smt_kernel.cpp b/src/smt/smt_kernel.cpp
index 4f48aec33..4f134ecac 100644
--- a/src/smt/smt_kernel.cpp
+++ b/src/smt/smt_kernel.cpp
@@ -232,6 +232,14 @@ namespace smt {
         void updt_params(params_ref const & p) {
             m_kernel.updt_params(p);
         }
+
+        void register_user_propagator(
+            void* ctx, 
+            std::function<void(void*, unsigned, expr*)>& fixed_eh,
+            std::function<void(void*)>&                  push_eh,
+            std::function<void(void*, unsigned)>&        pop_eh) {
+            m_kernel.register_user_propagator(ctx, fixed_eh, push_eh, pop_eh);
+        }
     };
 
     kernel::kernel(ast_manager & m, smt_params & fp, params_ref const & p) {
@@ -445,6 +453,14 @@ namespace smt {
         return m_imp->get_implied_upper_bound(e);
     }
 
+    void kernel::register_user_propagator(
+        void* ctx, 
+        std::function<void(void*, unsigned, expr*)>& fixed_eh,
+        std::function<void(void*)>&                  push_eh,
+        std::function<void(void*, unsigned)>&        pop_eh) {
+        m_imp->register_user_propagator(ctx, fixed_eh, push_eh, pop_eh);
+    }
+
 
 
 };
diff --git a/src/smt/smt_kernel.h b/src/smt/smt_kernel.h
index b9f80ac01..c14ba66bf 100644
--- a/src/smt/smt_kernel.h
+++ b/src/smt/smt_kernel.h
@@ -284,6 +284,15 @@ namespace smt {
         */
         static void collect_param_descrs(param_descrs & d);
 
+        /**
+           \brief register a user-propagator "theory"
+        */
+        void register_user_propagator(
+            void* ctx, 
+            std::function<void(void*, unsigned, expr*)>& fixed_eh,
+            std::function<void(void*)>&                  push_eh,
+            std::function<void(void*, unsigned)>&        pop_eh);
+
         /**
            \brief Return a reference to smt::context.
            This is a temporary hack to support user theories.
diff --git a/src/smt/smt_solver.cpp b/src/smt/smt_solver.cpp
index 495448eeb..62e7947d7 100644
--- a/src/smt/smt_solver.cpp
+++ b/src/smt/smt_solver.cpp
@@ -208,6 +208,15 @@ namespace {
             return m_context.get_trail();
         }
 
+        void register_user_propagator(
+            void* ctx, 
+            std::function<void(void*, unsigned, expr*)>& fixed_eh,
+            std::function<void(void*)>&                  push_eh,
+            std::function<void(void*, unsigned)>&        pop_eh) override {
+            m_context.register_user_propagator(ctx, fixed_eh, push_eh, pop_eh);
+        }
+
+
         struct scoped_minimize_core {
             smt_solver& s;
             expr_ref_vector m_assumptions;
diff --git a/src/smt/smt_theory.cpp b/src/smt/smt_theory.cpp
index 525285466..04a29e904 100644
--- a/src/smt/smt_theory.cpp
+++ b/src/smt/smt_theory.cpp
@@ -150,6 +150,18 @@ namespace smt {
         return lit;
     }
 
+    literal theory::mk_literal(expr* _e) {
+        expr_ref e(_e, m);
+        bool is_not = m.is_not(_e, _e);
+        if (!ctx.e_internalized(_e)) {
+            ctx.internalize(_e, is_quantifier(_e));
+        }
+        literal lit = ctx.get_literal(_e);
+        ctx.mark_as_relevant(lit);
+        if (is_not) lit.neg();
+        return lit;
+    }
+
     enode* theory::ensure_enode(expr* e) {
         if (!ctx.e_internalized(e)) {
             ctx.internalize(e, is_quantifier(e));
diff --git a/src/smt/smt_theory.h b/src/smt/smt_theory.h
index 991dbd5d7..61dc97c82 100644
--- a/src/smt/smt_theory.h
+++ b/src/smt/smt_theory.h
@@ -533,6 +533,8 @@ namespace smt {
 
         literal mk_preferred_eq(expr* a, expr* b);
 
+        literal mk_literal(expr* e);
+
         enode* ensure_enode(expr* e);
 
         enode* get_root(expr* e) { return ensure_enode(e)->get_root(); }
diff --git a/src/smt/theory_bv.cpp b/src/smt/theory_bv.cpp
index c0218fdf8..033df7aa0 100644
--- a/src/smt/theory_bv.cpp
+++ b/src/smt/theory_bv.cpp
@@ -465,6 +465,11 @@ namespace smt {
     void theory_bv::fixed_var_eh(theory_var v) {
         numeral val;
         VERIFY(get_fixed_value(v, val));
+        enode* n = get_enode(v);
+        if (ctx.watches_fixed(n)) {
+            expr_ref num(m_util.mk_numeral(val, m.get_sort(n->get_owner())), m);
+            ctx.assign_fixed(n, num, m_bits[v]);
+        }
         unsigned sz = get_bv_size(v);
         value_sort_pair key(val, sz);
         theory_var v2;
@@ -528,8 +533,8 @@ namespace smt {
     void theory_bv::internalize_num(app * n) {
         SASSERT(!ctx.e_internalized(n));
         numeral val;
-        unsigned sz;
-        m_util.is_numeral(n, val, sz);
+        unsigned sz = 0;
+        VERIFY(m_util.is_numeral(n, val, sz));
         enode * e    = mk_enode(n);
         // internalizer is marking enodes as interpreted whenever the associated ast is a value and a constant.
         // e->mark_as_interpreted();
diff --git a/src/smt/theory_seq.cpp b/src/smt/theory_seq.cpp
index f1ef7832d..c1e182aa2 100644
--- a/src/smt/theory_seq.cpp
+++ b/src/smt/theory_seq.cpp
@@ -2785,18 +2785,6 @@ literal theory_seq::mk_simplified_literal(expr * _e) {
     return mk_literal(e);
 }
 
-literal theory_seq::mk_literal(expr* _e) {
-    expr_ref e(_e, m);
-    bool is_not = m.is_not(_e, _e);
-    if (!ctx.e_internalized(_e)) {
-        ctx.internalize(_e, is_quantifier(_e));
-    }
-    literal lit = ctx.get_literal(_e);
-    ctx.mark_as_relevant(lit);
-    if (is_not) lit.neg();
-    return lit;
-}
-
 literal theory_seq::mk_seq_eq(expr* a, expr* b) {
     SASSERT(m_util.is_seq(a));
     return mk_literal(m_sk.mk_eq(a, b));
diff --git a/src/smt/theory_seq.h b/src/smt/theory_seq.h
index f0e5e06f2..f98eaf9cf 100644
--- a/src/smt/theory_seq.h
+++ b/src/smt/theory_seq.h
@@ -575,7 +575,6 @@ namespace smt {
 
         expr_ref add_elim_string_axiom(expr* n);
         void add_in_re_axiom(expr* n);
-        literal mk_literal(expr* n);
         literal mk_simplified_literal(expr* n);
         literal mk_eq_empty(expr* n, bool phase = true);
         literal mk_seq_eq(expr* a, expr* b);
diff --git a/src/smt/theory_str_mc.cpp b/src/smt/theory_str_mc.cpp
index 9b7df6c64..c8a6f3dcb 100644
--- a/src/smt/theory_str_mc.cpp
+++ b/src/smt/theory_str_mc.cpp
@@ -446,6 +446,10 @@ namespace smt {
         ast_manager & m = get_manager();
 
         ast_manager & sub_m = subsolver.m();
+
+        // NSB code review: to remove dependencies on subsolver.get_context(). 
+        // It uses a method that should be removed from smt_kernel.
+        // currently sub_ctx is used to retrieve a rewriter. Theory_str already has a rewriter attahed.
         context & sub_ctx = subsolver.get_context();
 
         expr * str = nullptr, *re = nullptr;
diff --git a/src/smt/user_propagator.cpp b/src/smt/user_propagator.cpp
new file mode 100644
index 000000000..c4c15481f
--- /dev/null
+++ b/src/smt/user_propagator.cpp
@@ -0,0 +1,85 @@
+/*++
+Copyright (c) 2020 Microsoft Corporation
+
+Module Name:
+
+    user_propagator.cpp
+
+Abstract:
+
+    User theory propagator plugin.
+
+Author:
+
+    Nikolaj Bjorner (nbjorner) 2020-08-17
+
+--*/
+
+
+#include "smt/user_propagator.h"
+#include "smt/smt_context.h"
+
+using namespace smt;
+
+user_propagator::user_propagator(context& ctx):
+    theory(ctx, ctx.get_manager().mk_family_id("user_propagator")),
+    m_qhead(0)
+{}
+
+unsigned user_propagator::add_expr(expr* e) {
+    return mk_var(ensure_enode(e));
+}
+
+void user_propagator::new_fixed_eh(theory_var v, expr* value, unsigned num_lits, literal const* jlits) {
+    m_id2justification.setx(v, literal_vector(num_lits, jlits), literal_vector());
+    m_fixed_eh(m_user_context, v, value);
+}
+
+void user_propagator::push_scope_eh() {
+    theory::push_scope_eh();
+    m_push_eh(m_user_context);
+    m_prop_lim.push_back(m_prop.size());
+}
+
+void user_propagator::pop_scope_eh(unsigned num_scopes) {
+    m_pop_eh(m_user_context, num_scopes);
+    theory::pop_scope_eh(num_scopes);
+    unsigned old_sz = m_prop_lim.size() - num_scopes;
+    m_prop.shrink(m_prop_lim[old_sz]);
+    m_prop_lim.shrink(old_sz);
+}
+
+bool user_propagator::can_propagate() {
+    return m_qhead < m_prop.size();
+}
+
+void user_propagator::propagate() {
+    unsigned qhead = m_qhead;
+    literal_vector lits;
+    enode_pair_vector eqs;
+    justification* js;
+    while (qhead < m_prop.size() && !ctx.inconsistent()) {
+        auto const& prop = m_prop[qhead];
+        lits.reset();        
+        for (unsigned id : prop.m_ids) 
+            lits.append(m_id2justification[id]);
+        if (m.is_false(prop.m_conseq)) {
+            js = ctx.mk_justification(
+                ext_theory_conflict_justification(
+                    get_id(), ctx.get_region(), lits.size(), lits.c_ptr(), eqs.size(), eqs.c_ptr(), 0, nullptr));
+            ctx.set_conflict(js);
+        }
+        else {
+            literal lit = mk_literal(prop.m_conseq);
+            js = ctx.mk_justification(
+                ext_theory_propagation_justification(
+                    get_id(), ctx.get_region(), lits.size(), lits.c_ptr(), eqs.size(), eqs.c_ptr(), lit));
+            ctx.assign(lit, js);
+        }
+        ++qhead;
+    }
+    ctx.push_trail(value_trail<context, unsigned>(m_qhead));
+    m_qhead = qhead;
+}
+
+
diff --git a/src/smt/user_propagator.h b/src/smt/user_propagator.h
new file mode 100644
index 000000000..4eaebc0d6
--- /dev/null
+++ b/src/smt/user_propagator.h
@@ -0,0 +1,97 @@
+/*++
+Copyright (c) 2020 Microsoft Corporation
+
+Module Name:
+
+    user_propagator.h
+
+Abstract:
+
+    User-propagator plugin.
+    Or, user-propagator in response to registered
+    terms receiveing fixed values.
+
+Author:
+
+    Nikolaj Bjorner (nbjorner) 2020-08-17
+
+Notes:
+
+- could also be complemented with disequalities to fixed values to narrow range of variables.
+
+
+--*/
+
+#pragma once
+
+#include "smt/smt_theory.h"
+
+namespace smt {
+    class user_propagator : public theory {
+        void* m_user_context;
+        std::function<void(void*, unsigned, expr*)> m_fixed_eh;
+        std::function<void(void*)>                  m_push_eh;
+        std::function<void(void*, unsigned)>        m_pop_eh;
+        struct prop_info {
+            unsigned_vector m_ids;
+            expr_ref        m_conseq;
+            prop_info(unsigned sz, unsigned const* ids, expr_ref const& c):
+                m_ids(sz, ids),
+                m_conseq(c)
+            {}
+        };
+        unsigned               m_qhead;
+        vector<prop_info>      m_prop;
+        unsigned_vector        m_prop_lim;
+        vector<literal_vector> m_id2justification;
+
+    public:
+        user_propagator(context& ctx);
+        
+        ~user_propagator() override {}
+
+        /*
+         * \brief initial setup for user propagator.
+         */
+        void add(
+            void* ctx, 
+            std::function<void(void*, unsigned, expr*)>& fixed_eh,
+            std::function<void(void*)>&                  push_eh,
+            std::function<void(void*, unsigned)>&        pop_eh) {
+            m_user_context = ctx;
+            m_fixed_eh     = fixed_eh;
+            m_push_eh      = push_eh;
+            m_pop_eh       = pop_eh;
+        }
+
+        unsigned add_expr(expr* e);
+
+        void add_propagation(unsigned sz, unsigned const* ids, expr* conseq) {
+            m_prop.push_back(prop_info(sz, ids, expr_ref(conseq, m)));
+        }
+
+        void new_fixed_eh(theory_var v, expr* value, unsigned num_lits, literal const* jlits);
+
+        theory * mk_fresh(context * new_ctx) override { UNREACHABLE(); return alloc(user_propagator, *new_ctx); }
+        bool internalize_atom(app * atom, bool gate_ctx) override { UNREACHABLE(); return false; }
+        bool internalize_term(app * term) override { UNREACHABLE(); return false; }
+        void new_eq_eh(theory_var v1, theory_var v2) override { UNREACHABLE(); }
+        void new_diseq_eh(theory_var v1, theory_var v2) override { UNREACHABLE(); }
+        bool use_diseqs() const override { return false; }
+        bool build_models() const override { return false; }
+        final_check_status final_check_eh() override { UNREACHABLE(); return FC_DONE; }
+        void reset_eh() override {}
+        void assign_eh(bool_var v, bool is_true) override { UNREACHABLE(); }
+        void init_search_eh() override {}
+        void push_scope_eh() override;
+        void pop_scope_eh(unsigned num_scopes) override;
+        void restart_eh() override {}
+        void collect_statistics(::statistics & st) const override {}
+        model_value_proc * mk_value(enode * n, model_generator & mg) override { return nullptr; }
+        void init_model(model_generator & m) override {}
+        bool include_func_interp(func_decl* f) override { return false; }
+        bool can_propagate() override;
+        void propagate() override; 
+        void display(std::ostream& out) const {}
+    };
+};
diff --git a/src/solver/solver.h b/src/solver/solver.h
index e3d980021..6adefc6e9 100644
--- a/src/solver/solver.h
+++ b/src/solver/solver.h
@@ -238,6 +238,14 @@ public:
 
     virtual expr_ref get_implied_upper_bound(expr* e) = 0;
 
+    virtual void register_user_propagator(
+        void* ctx, 
+        std::function<void(void*, unsigned, expr*)>& fixed_eh,
+        std::function<void(void*)>&                  push_eh,
+        std::function<void(void*, unsigned)>&        pop_eh) {
+        throw default_exception("user-propagators are only supported on the SMT solver");
+    }
+
 
     /**
        \brief Display the content of this solver.