diff --git a/src/muz/spacer/spacer_unsat_core_learner.cpp b/src/muz/spacer/spacer_unsat_core_learner.cpp
index da0d6ec34..08f7f7ce8 100644
--- a/src/muz/spacer/spacer_unsat_core_learner.cpp
+++ b/src/muz/spacer/spacer_unsat_core_learner.cpp
@@ -36,28 +36,27 @@ void unsat_core_learner::register_plugin(unsat_core_plugin* plugin) {
 }
 
 void unsat_core_learner::compute_unsat_core(expr_ref_vector& unsat_core) {
-    // traverse proof
     proof_post_order it(m_pr.get(), m);
     while (it.hasNext()) {
-        proof* currentNode = it.next();
+        proof* curr = it.next();
 
-        if (m.get_num_parents(currentNode) > 0) {
-            bool need_to_mark_closed = true;
+        bool done = is_closed(curr);
+        if (done) continue;
 
-            for (proof* premise : m.get_parents(currentNode)) {
-                need_to_mark_closed &= (!m_pr.is_b_marked(premise) || m_closed.is_marked(premise));
-            }
-
-            // save result
-            m_closed.mark(currentNode, need_to_mark_closed);
+        if (m.get_num_parents(curr) > 0) {
+            done = true;
+            for (proof* p : m.get_parents(curr)) done &= !is_b_open(p);
+            set_closed(curr, done);
         }
 
-        // we have now collected all necessary information, so we can visit the node
-        // if the node mixes A-reasoning and B-reasoning and contains non-closed premises
-        if (m_pr.is_a_marked(currentNode) && 
-            m_pr.is_b_marked(currentNode) && 
-            !m_closed.is_marked(currentNode)) {
-            compute_partial_core(currentNode); // then we need to compute a partial core
+        // we have now collected all necessary information,
+        // so we can visit the node
+        // if the node mixes A-reasoning and B-reasoning
+        // and contains non-closed premises
+        if (!done) {
+            if (m_pr.is_a_marked(curr) && m_pr.is_b_marked(curr)) {
+                compute_partial_core(curr);
+            }
         }
     }
 
@@ -74,7 +73,7 @@ void unsat_core_learner::compute_unsat_core(expr_ref_vector& unsat_core) {
 
 void unsat_core_learner::compute_partial_core(proof* step) {
     for (unsat_core_plugin* plugin : m_plugins) {
-        if (m_closed.is_marked(step)) break;
+        if (is_closed(step)) break;
         plugin->compute_partial_core(step);
     }
 }
diff --git a/src/muz/spacer/spacer_unsat_core_learner.h b/src/muz/spacer/spacer_unsat_core_learner.h
index 327b12eb6..42ad71501 100644
--- a/src/muz/spacer/spacer_unsat_core_learner.h
+++ b/src/muz/spacer/spacer_unsat_core_learner.h
@@ -22,6 +22,7 @@ Revision History:
 #include "ast/ast.h"
 #include "muz/spacer/spacer_util.h"
 #include "muz/spacer/spacer_proof_utils.h"
+#include "muz/spacer/spacer_iuc_proof.h"
 
 namespace spacer {
 
@@ -31,13 +32,25 @@ namespace spacer {
     class unsat_core_learner {
         typedef obj_hashtable<expr> expr_set;
 
+        ast_manager& m;
+        iuc_proof&   m_pr;
+
+        ptr_vector<unsat_core_plugin> m_plugins;
+        ast_mark m_closed;
+
+        expr_ref_vector m_unsat_core;
+
     public:
         unsat_core_learner(ast_manager& m, iuc_proof& pr) :
             m(m), m_pr(pr), m_unsat_core(m) {};
         virtual ~unsat_core_learner();
 
-        ast_manager& m;
-        iuc_proof&   m_pr;
+        ast_manager& get_manager() {return m;}
+
+        bool is_a(proof *pr) {return m_pr.is_a_marked(pr);}
+        bool is_b(proof *pr) {return m_pr.is_b_marked(pr);}
+        bool is_h(proof *pr) {return m_pr.is_h_marked(pr);}
+        bool is_b_pure(proof *pr) { return m_pr.is_b_pure(pr);}
 
         /*
          * register a plugin for computation of partial unsat cores
@@ -67,14 +80,6 @@ namespace spacer {
         void add_lemma_to_core(expr* lemma);
 
     private:
-        ptr_vector<unsat_core_plugin> m_plugins;
-        ast_mark m_closed;
-
-        /*
-         * collects the lemmas of the unsat-core
-         * will at the end be inserted into unsat_core.
-         */
-        expr_ref_vector m_unsat_core;
 
         /*
          * computes partial core for step by delegating computation to plugins
diff --git a/src/muz/spacer/spacer_unsat_core_plugin.cpp b/src/muz/spacer/spacer_unsat_core_plugin.cpp
index 79bafdfeb..ef97d81df 100644
--- a/src/muz/spacer/spacer_unsat_core_plugin.cpp
+++ b/src/muz/spacer/spacer_unsat_core_plugin.cpp
@@ -34,27 +34,25 @@ Revision History:
 
 namespace spacer {
 
-    unsat_core_plugin::unsat_core_plugin(unsat_core_learner& learner):
-        m(learner.m), m_learner(learner) {};
+    unsat_core_plugin::unsat_core_plugin(unsat_core_learner& ctx):
+        m(ctx.get_manager()), m_ctx(ctx) {};
 
     void unsat_core_plugin_lemma::compute_partial_core(proof* step) {
-        SASSERT(m_learner.m_pr.is_a_marked(step));
-        SASSERT(m_learner.m_pr.is_b_marked(step));
+        SASSERT(m_ctx.is_a(step));
+        SASSERT(m_ctx.is_b(step));
 
-        for (proof* premise : m.get_parents(step)) {
-
-            if (m_learner.is_b_open (premise))  {
+        for (auto* p : m.get_parents(step)) {
+            if (m_ctx.is_b_open (p))  {
                 // by IH, premises that are AB marked are already closed
-                SASSERT(!m_learner.m_pr.is_a_marked(premise));
-                add_lowest_split_to_core(premise);
+                SASSERT(!m_ctx.is_a(p));
+                add_lowest_split_to_core(p);
             }
         }
-        m_learner.set_closed(step, true);
+        m_ctx.set_closed(step, true);
     }
 
-    void unsat_core_plugin_lemma::add_lowest_split_to_core(proof* step) const
-    {
-        SASSERT(m_learner.is_b_open(step));
+    void unsat_core_plugin_lemma::add_lowest_split_to_core(proof* step) const {
+        SASSERT(m_ctx.is_b_open(step));
 
         ptr_buffer<proof> todo;
         todo.push_back(step);
@@ -64,44 +62,45 @@ namespace spacer {
             todo.pop_back();
 
             // if current step hasn't been processed,
-            if (!m_learner.is_closed(pf)) {
-                m_learner.set_closed(pf, true);
+            if (!m_ctx.is_closed(pf)) {
+                m_ctx.set_closed(pf, true);
                 // the step is b-marked and not closed.
                 // by I.H. the step must be already visited
                 // so if it is also a-marked, it must be closed
-                SASSERT(m_learner.m_pr.is_b_marked(pf));
-                SASSERT(!m_learner.m_pr.is_a_marked(pf));
+                SASSERT(m_ctx.is_b(pf));
+                SASSERT(!m_ctx.is_a(pf));
 
                 // the current step needs to be interpolated:
                 expr* fact = m.get_fact(pf);
                 // if we trust the current step and we are able to use it
-                if (m_learner.m_pr.is_b_pure (pf) &&
-                    (m.is_asserted(pf) || is_literal(m, fact))) {
+                if (m_ctx.is_b_pure (pf) && (m.is_asserted(pf) || is_literal(m, fact))) {
                     // just add it to the core
-                    m_learner.add_lemma_to_core(fact);
+                    m_ctx.add_lemma_to_core(fact);
                 }
                 // otherwise recurse on premises
                 else {
                     for (proof* premise : m.get_parents(pf))
-                        if (m_learner.is_b_open(premise))
+                        if (m_ctx.is_b_open(premise))
                             todo.push_back(premise);
                 }
-
             }
         }
     }
 
 
-    void unsat_core_plugin_farkas_lemma::compute_partial_core(proof* step)
-    {
-        SASSERT(m_learner.m_pr.is_a_marked(step));
-        SASSERT(m_learner.m_pr.is_b_marked(step));
+    /***
+     * FARKAS
+     */
+    void unsat_core_plugin_farkas_lemma::compute_partial_core(proof* step) {
+        SASSERT(m_ctx.is_a(step));
+        SASSERT(m_ctx.is_b(step));
         // XXX this assertion should be true so there is no need to check for it
-        SASSERT (!m_learner.is_closed (step));
+        SASSERT (!m_ctx.is_closed (step));
         func_decl* d = step->get_decl();
         symbol sym;
-        if (!m_learner.is_closed(step) && // if step is not already interpolated
-            is_farkas_lemma(m, step)) {
+        TRACE("spacer.farkas",
+              tout << "looking at: " << mk_pp(step, m) << "\n";);
+        if (!m_ctx.is_closed(step) && is_farkas_lemma(m, step)) {
             // weaker check : d->get_num_parameters() >= m.get_num_parents(step) + 2
 
             SASSERT(d->get_num_parameters() == m.get_num_parents(step) + 2);
@@ -136,24 +135,24 @@ namespace spacer {
             parameter const* params = d->get_parameters() + 2; // point to the first Farkas coefficient
 
             TRACE("spacer.farkas",
-                   tout << "Farkas input: "<< "\n";
-                   for (unsigned i = 0; i < m.get_num_parents(step); ++i) {
-                       proof * prem = m.get_parent(step, i);
-                       rational coef = params[i].get_rational();
-                       bool b_pure = m_learner.m_pr.is_b_pure (prem);
-                       tout << (b_pure?"B":"A") << " " << coef << " " << mk_pp(m.get_fact(prem), m) << "\n";
-                   }
-                   );
+                  tout << "Farkas input: "<< "\n";
+                  for (unsigned i = 0; i < m.get_num_parents(step); ++i) {
+                      proof * prem = m.get_parent(step, i);
+                      rational coef = params[i].get_rational();
+                      bool b_pure = m_ctx.is_b_pure (prem);
+                      tout << (b_pure?"B":"A") << " " << coef << " " << mk_pp(m.get_fact(prem), m) << "\n";
+                  }
+                );
 
-            bool can_be_closed = true;
+            bool done = true;
 
             for (unsigned i = 0; i < m.get_num_parents(step); ++i) {
                 proof * premise = m.get_parent(step, i);
 
-                if (m_learner.is_b_open (premise)) {
-                    SASSERT(!m_learner.m_pr.is_a_marked(premise));
+                if (m_ctx.is_b_open (premise)) {
+                    SASSERT(!m_ctx.is_a(premise));
 
-                    if (m_learner.m_pr.is_b_pure (premise)) {
+                    if (m_ctx.is_b_pure (premise)) {
                         if (!m_use_constant_from_a) {
                             rational coefficient = params[i].get_rational();
                             coeff_lits.push_back(std::make_pair(abs(coefficient), (app*)m.get_fact(premise)));
@@ -161,7 +160,7 @@ namespace spacer {
                     }
                     else {
                         // -- mixed premise, won't be able to close this proof step
-                        can_be_closed = false;
+                        done = false;
 
                         if (m_use_constant_from_a) {
                             rational coefficient = params[i].get_rational();
@@ -177,6 +176,7 @@ namespace spacer {
                 }
             }
 
+            // TBD: factor into another method
             if (m_use_constant_from_a) {
                 params += m.get_num_parents(step); // point to the first Farkas coefficient, which corresponds to a formula in the conclusion
 
@@ -208,11 +208,9 @@ namespace spacer {
             // only if all b-premises can be used directly, add the farkas core and close the step
             // AG: this decision needs to be re-evaluated. If the proof cannot be closed, literals above
             // AG: it will go into the core. However, it does not mean that this literal should/could not be added.
-            if (can_be_closed) {
-                m_learner.set_closed(step, true);
-                expr_ref res = compute_linear_combination(coeff_lits);
-                m_learner.add_lemma_to_core(res);
-            }
+            m_ctx.set_closed(step, done);
+            expr_ref res = compute_linear_combination(coeff_lits);
+            m_ctx.add_lemma_to_core(res);
         }
     }
 
@@ -236,12 +234,12 @@ namespace spacer {
 
     void unsat_core_plugin_farkas_lemma_optimized::compute_partial_core(proof* step)
     {
-        SASSERT(m_learner.m_pr.is_a_marked(step));
-        SASSERT(m_learner.m_pr.is_b_marked(step));
+        SASSERT(m_ctx.is_a(step));
+        SASSERT(m_ctx.is_b(step));
 
         func_decl* d = step->get_decl();
         symbol sym;
-        if (!m_learner.is_closed(step) && // if step is not already interpolated
+        if (!m_ctx.is_closed(step) && // if step is not already interpolated
            is_farkas_lemma(m, step)) {
             SASSERT(d->get_num_parameters() == m.get_num_parents(step) + 2);
             SASSERT(m.has_fact(step));
@@ -250,25 +248,25 @@ namespace spacer {
 
             parameter const* params = d->get_parameters() + 2; // point to the first Farkas coefficient
 
-            STRACE("spacer.farkas",
-                   verbose_stream() << "Farkas input: "<< "\n";
-                   for (unsigned i = 0; i < m.get_num_parents(step); ++i) {
-                       proof * prem = m.get_parent(step, i);
-                       rational coef = params[i].get_rational();
-                       bool b_pure = m_learner.m_pr.is_b_pure (prem);
-                       verbose_stream() << (b_pure?"B":"A") << " " << coef << " " << mk_pp(m.get_fact(prem), m_learner.m) << "\n";
-                   }
-                   );
+            TRACE("spacer.farkas",
+                  tout << "Farkas input: "<< "\n";
+                  for (unsigned i = 0; i < m.get_num_parents(step); ++i) {
+                      proof * prem = m.get_parent(step, i);
+                      rational coef = params[i].get_rational();
+                      bool b_pure = m_ctx.is_b_pure (prem);
+                      tout << (b_pure?"B":"A") << " " << coef << " " << mk_pp(m.get_fact(prem), m) << "\n";
+                  }
+                );
 
             bool can_be_closed = true;
             for (unsigned i = 0; i < m.get_num_parents(step); ++i) {
                 proof * premise = m.get_parent(step, i);
 
-                if (m_learner.m_pr.is_b_marked(premise) && !m_learner.is_closed(premise))
+                if (m_ctx.is_b(premise) && !m_ctx.is_closed(premise))
                 {
-                    SASSERT(!m_learner.m_pr.is_a_marked(premise));
+                    SASSERT(!m_ctx.is_a(premise));
 
-                    if (m_learner.m_pr.is_b_pure(premise))
+                    if (m_ctx.is_b_pure(premise))
                     {
                         rational coefficient = params[i].get_rational();
                         linear_combination.push_back
@@ -284,7 +282,7 @@ namespace spacer {
             // only if all b-premises can be used directly, close the step and add linear combinations for later processing
             if (can_be_closed)
             {
-                m_learner.set_closed(step, true);
+                m_ctx.set_closed(step, true);
                 if (!linear_combination.empty())
                 {
                     m_linear_combinations.push_back(linear_combination);
@@ -350,7 +348,7 @@ namespace spacer {
             SASSERT(!coeff_lits.empty());
             expr_ref linear_combination = compute_linear_combination(coeff_lits);
 
-            m_learner.add_lemma_to_core(linear_combination);
+            m_ctx.add_lemma_to_core(linear_combination);
         }
 
     }
@@ -481,7 +479,7 @@ namespace spacer {
                     SASSERT(!coeff_lits.empty()); // since then previous outer loop would have found solution already
                     expr_ref linear_combination = compute_linear_combination(coeff_lits);
 
-                    m_learner.add_lemma_to_core(linear_combination);
+                    m_ctx.add_lemma_to_core(linear_combination);
                 }
                 return;
             }
@@ -505,10 +503,10 @@ namespace spacer {
     {
         ptr_vector<proof> todo;
 
-        SASSERT(m_learner.m_pr.is_a_marked(step));
-        SASSERT(m_learner.m_pr.is_b_marked(step));
+        SASSERT(m_ctx.is_a(step));
+        SASSERT(m_ctx.is_b(step));
         SASSERT(m.get_num_parents(step) > 0);
-        SASSERT(!m_learner.is_closed(step));
+        SASSERT(!m_ctx.is_closed(step));
         todo.push_back(step);
 
         while (!todo.empty())
@@ -517,7 +515,7 @@ namespace spacer {
             todo.pop_back();
 
             // if we need to deal with the node and if we haven't added the corresponding edges so far
-            if (!m_learner.is_closed(current) && !m_visited.is_marked(current))
+            if (!m_ctx.is_closed(current) && !m_visited.is_marked(current))
             {
                 // compute smallest subproof rooted in current, which has only good edges
                 // add an edge from current to each leaf of that subproof
@@ -528,7 +526,7 @@ namespace spacer {
 
             }
         }
-        m_learner.set_closed(step, true);
+        m_ctx.set_closed(step, true);
     }
 
 
@@ -539,7 +537,7 @@ namespace spacer {
         ptr_buffer<proof> todo_subproof;
 
         for (proof* premise : m.get_parents(step)) {
-            if (m_learner.m_pr.is_b_marked(premise)) {
+            if (m_ctx.is_b(premise)) {
                 todo_subproof.push_back(premise);
             }
         }
@@ -549,21 +547,21 @@ namespace spacer {
             todo_subproof.pop_back();
 
             // if we need to deal with the node
-            if (!m_learner.is_closed(current))
+            if (!m_ctx.is_closed(current))
             {
-                SASSERT(!m_learner.m_pr.is_a_marked(current)); // by I.H. the step must be already visited
+                SASSERT(!m_ctx.is_a(current)); // by I.H. the step must be already visited
 
                 // and the current step needs to be interpolated:
-                if (m_learner.m_pr.is_b_marked(current))
+                if (m_ctx.is_b(current))
                 {
                     // if we trust the current step and we are able to use it
-                    if (m_learner.m_pr.is_b_pure (current) &&
+                    if (m_ctx.is_b_pure (current) &&
                         (m.is_asserted(current) ||
                          is_literal(m, m.get_fact(current))))
                     {
                         // we found a leaf of the subproof, so
                         // 1) we add corresponding edges
-                        if (m_learner.m_pr.is_a_marked(step))
+                        if (m_ctx.is_a(step))
                         {
                             add_edge(nullptr, current); // current is sink
                         }
@@ -685,7 +683,7 @@ namespace spacer {
         m_min_cut.compute_min_cut(cut_nodes);
 
         for (unsigned cut_node : cut_nodes)   {
-            m_learner.add_lemma_to_core(m_node_to_formula[cut_node]);
+            m_ctx.add_lemma_to_core(m_node_to_formula[cut_node]);
         }
     }
 }
diff --git a/src/muz/spacer/spacer_unsat_core_plugin.h b/src/muz/spacer/spacer_unsat_core_plugin.h
index c05bcc5b9..746f21b19 100644
--- a/src/muz/spacer/spacer_unsat_core_plugin.h
+++ b/src/muz/spacer/spacer_unsat_core_plugin.h
@@ -35,14 +35,14 @@ namespace spacer {
         virtual ~unsat_core_plugin() {};
         virtual void compute_partial_core(proof* step) = 0;
         virtual void finalize(){};
-        
-        unsat_core_learner& m_learner;
+
+        unsat_core_learner& m_ctx;
     };
 
-    class unsat_core_plugin_lemma : public unsat_core_plugin {        
+    class unsat_core_plugin_lemma : public unsat_core_plugin {
     public:
-        unsat_core_plugin_lemma(unsat_core_learner& learner) : unsat_core_plugin(learner){};        
-        void compute_partial_core(proof* step) override;        
+        unsat_core_plugin_lemma(unsat_core_learner& learner) : unsat_core_plugin(learner){};
+        void compute_partial_core(proof* step) override;
     private:
         void add_lowest_split_to_core(proof* step) const;
     };
@@ -54,7 +54,7 @@ namespace spacer {
                                        bool use_constant_from_a=true) :
             unsat_core_plugin(learner),
             m_split_literals(split_literals),
-            m_use_constant_from_a(use_constant_from_a) {};        
+            m_use_constant_from_a(use_constant_from_a) {};
         void compute_partial_core(proof* step) override;
     private:
         bool m_split_literals;
@@ -64,8 +64,8 @@ namespace spacer {
          */
         expr_ref compute_linear_combination(const coeff_lits_t& coeff_lits);
     };
-    
-    class unsat_core_plugin_farkas_lemma_optimized : public unsat_core_plugin {        
+
+    class unsat_core_plugin_farkas_lemma_optimized : public unsat_core_plugin {
     public:
         unsat_core_plugin_farkas_lemma_optimized(unsat_core_learner& learner, ast_manager& m) : unsat_core_plugin(learner) {};
         void compute_partial_core(proof* step) override;