diff --git a/src/sat/sat_aig_cuts.cpp b/src/sat/sat_aig_cuts.cpp
index 15bb3f268..c1bf18ac7 100644
--- a/src/sat/sat_aig_cuts.cpp
+++ b/src/sat/sat_aig_cuts.cpp
@@ -43,11 +43,11 @@ namespace sat {
             if (m_aig[id].empty()) {
                 continue;
             }
-            IF_VERBOSE(3, m_cuts[id].display(verbose_stream() << "augment " << id << "\nbefore\n"));
+            IF_VERBOSE(10, m_cuts[id].display(verbose_stream() << "augment " << id << "\nbefore\n"));
             for (node const& n : m_aig[id]) {
                 augment(id, n);
             }
-            IF_VERBOSE(3, m_cuts[id].display(verbose_stream() << "after\n"));            
+            IF_VERBOSE(10, m_cuts[id].display(verbose_stream() << "after\n"));            
         }
     }
 
@@ -82,7 +82,7 @@ namespace sat {
     }
 
     bool aig_cuts::insert_cut(unsigned v, cut const& c, cut_set& cs) {
-        if (!cs.insert(&m_on_cut_add, &m_on_cut_del, c)) {
+        if (!cs.insert(m_on_cut_add, m_on_cut_del, c)) {
             return true;
         }
         m_num_cuts++;
@@ -98,7 +98,7 @@ namespace sat {
     }
 
     void aig_cuts::augment_ite(unsigned v, node const& n, cut_set& cs) {
-        IF_VERBOSE(2, display(verbose_stream() << "augment_ite " << v << " ", n) << "\n");
+        IF_VERBOSE(4, display(verbose_stream() << "augment_ite " << v << " ", n) << "\n");
         literal l1 = child(n, 0);
         literal l2 = child(n, 1);
         literal l3 = child(n, 2);
@@ -172,7 +172,7 @@ namespace sat {
 
     void aig_cuts::augment_aigN(unsigned v, node const& n, cut_set& cs) {
         IF_VERBOSE(4, display(verbose_stream() << "augment_aigN " << v << " ", n) << "\n");
-        m_cut_set1.reset(nullptr);
+        m_cut_set1.reset(m_on_cut_del);
         SASSERT(n.is_and() || n.is_xor());
         literal lit = child(n, 0);
         for (auto const& a : m_cuts[lit.var()]) {
@@ -180,10 +180,10 @@ namespace sat {
             if (lit.sign()) {
                 b.negate();
             }            
-            m_cut_set1.push_back(nullptr, b);
+            m_cut_set1.push_back(m_on_cut_add, b);
         }
         for (unsigned i = 1; i < n.size(); ++i) {
-            m_cut_set2.reset(nullptr);
+            m_cut_set2.reset(m_on_cut_del);
             lit = child(n, i);
             m_insertions = 0;
             for (auto const& a : m_cut_set1) {
@@ -212,6 +212,12 @@ namespace sat {
         }        
     }
 
+    void aig_cuts::replace(unsigned v, cut const& src, cut const& dst) {
+        m_cuts[v].replace(m_on_cut_add, m_on_cut_del, src, dst);
+        touch(v);
+    }
+
+
     bool aig_cuts::is_touched(node const& n) {
         for (unsigned i = 0; i < n.size(); ++i) {
             literal lit = m_literals[n.offset() + i];
diff --git a/src/sat/sat_aig_cuts.h b/src/sat/sat_aig_cuts.h
index 2fd1fcfc0..feefb7ec8 100644
--- a/src/sat/sat_aig_cuts.h
+++ b/src/sat/sat_aig_cuts.h
@@ -138,10 +138,10 @@ namespace sat {
         void on_node_add(unsigned v, node const& n);
         void on_node_del(unsigned v, node const& n);
 
-        void evict(cut_set& cs, unsigned idx) { cs.evict(&m_on_cut_del, idx); }
-        void reset(cut_set& cs) { cs.reset(&m_on_cut_del); }
-        void push_back(cut_set& cs, cut const& c) { cs.push_back(&m_on_cut_add, c); }
-        void shrink(cut_set& cs, unsigned j) { cs.shrink(&m_on_cut_del, j); }
+        void evict(cut_set& cs, unsigned idx) { cs.evict(m_on_cut_del, idx); }
+        void reset(cut_set& cs) { cs.reset(m_on_cut_del); }
+        void push_back(cut_set& cs, cut const& c) { cs.push_back(m_on_cut_add, c); }
+        void shrink(cut_set& cs, unsigned j) { cs.shrink(m_on_cut_del, j); }
 
         void cut2clauses(on_clause_t& on_clause, unsigned v, cut const& c);
         void node2def(on_clause_t& on_clause, node const& n, literal r);
@@ -166,6 +166,8 @@ namespace sat {
 
         void cut2def(on_clause_t& on_clause, cut const& c, literal r);
 
+        void replace(unsigned v, cut const& src, cut const& dst);
+
 
         std::ostream& display(std::ostream& out) const;
 
diff --git a/src/sat/sat_aig_simplifier.cpp b/src/sat/sat_aig_simplifier.cpp
index edac66071..a94e131dd 100644
--- a/src/sat/sat_aig_simplifier.cpp
+++ b/src/sat/sat_aig_simplifier.cpp
@@ -68,8 +68,9 @@ namespace sat {
             for (literal lit : clause) m_assumptions.push_back(~lit);           
             lbool r = s.check(clause.size(), m_assumptions.c_ptr());
             if (r != l_false) {
-                std::cout << "not validated: " << clause << "\n";
-                s.display(std::cout);
+                IF_VERBOSE(0, 
+                           verbose_stream() << "not validated: " << clause << "\n";
+                           s.display(verbose_stream()););
                 std::string line;
                 std::getline(std::cin, line);                
             }
@@ -78,7 +79,6 @@ namespace sat {
 
     void aig_simplifier::ensure_validator() {
         if (!m_validator) {
-            std::cout << "init validator\n";
             params_ref p;
             p.set_bool("aig", false);
             p.set_bool("drat.check_unsat", false);
@@ -92,15 +92,9 @@ namespace sat {
         s(_s), 
         m_trail_size(0),
         m_validator(nullptr) {  
-        if (false) {
-            ensure_validator();
-            std::function<void(literal_vector const& clause)> _on_add = 
-                [this](literal_vector const& clause) { 
-                std::cout << "add " << clause << "\n"; m_validator->validate(clause); 
-            };
-            m_aig_cuts.set_on_clause_add(_on_add);
-        }
-        else if (s.get_config().m_drat) {
+        m_config.m_enable_dont_cares = true;
+        m_config.m_enable_units = true;
+        if (s.get_config().m_drat) {
             std::function<void(literal_vector const& clause)> _on_add = 
                 [this](literal_vector const& clause) { s.m_drat.add(clause); };
             std::function<void(literal_vector const& clause)> _on_del = 
@@ -108,6 +102,15 @@ namespace sat {
             m_aig_cuts.set_on_clause_add(_on_add);
             m_aig_cuts.set_on_clause_del(_on_del);
         }
+        else if (m_config.m_validate) {
+            ensure_validator();
+            std::function<void(literal_vector const& clause)> _on_add = 
+                [this](literal_vector const& clause) { 
+                m_validator->validate(clause); 
+            };
+            m_aig_cuts.set_on_clause_add(_on_add);
+        }
+
     }
 
     aig_simplifier::~aig_simplifier() {
@@ -158,7 +161,7 @@ namespace sat {
         ++m_stats.m_num_calls;
         do {
             n = m_stats.m_num_eqs + m_stats.m_num_units;
-            if (m_config.m_full || true) clauses2aig();
+            clauses2aig();
             aig2clauses();
             ++i;
         }
@@ -172,7 +175,7 @@ namespace sat {
     void aig_simplifier::clauses2aig() {
 
         // update units
-        for (; m_config.m_full && m_trail_size < s.init_trail_size(); ++m_trail_size) {
+        for (; m_config.m_enable_units && m_trail_size < s.init_trail_size(); ++m_trail_size) {
             literal lit = s.trail_literal(m_trail_size);
             m_aig_cuts.add_node(lit, and_op, 0, 0);
         }
@@ -192,7 +195,7 @@ namespace sat {
         af.set(on_and);
         af.set(on_ite);
         clause_vector clauses(s.clauses());
-        if (m_config.m_full || true) clauses.append(s.learned());
+        if (m_config.m_add_learned) clauses.append(s.learned());
         af(clauses);
 
         std::function<void (literal_vector const&)> on_xor = 
@@ -229,6 +232,8 @@ namespace sat {
         vector<cut_set> const& cuts = m_aig_cuts();
         m_stats.m_num_cuts = m_aig_cuts.num_cuts();
 
+        add_dont_cares(cuts);
+
         map<cut const*, unsigned, cut::hash_proc, cut::eq_proc> cut2id;
                 
         union_find_default_ctx ctx;
@@ -242,20 +247,20 @@ namespace sat {
         for (unsigned i = cuts.size(); i-- > 0; ) {
             for (auto& c : cuts[i]) {
                 unsigned j = 0;
-                if (m_config.m_full && c.is_true()) {
+                if (m_config.m_enable_units && c.is_true()) {
                     if (s.value(i) == l_undef) {
                         literal lit(i, false);
-                        validate_unit(lit);
+                        // validate_unit(lit);
                         IF_VERBOSE(2, verbose_stream() << "new unit " << lit << "\n");
                         s.assign_unit(lit);
                         ++m_stats.m_num_units;
                     }
                     break;
                 }
-                if (m_config.m_full && c.is_false()) {
+                if (m_config.m_enable_units && c.is_false()) {
                     if (s.value(i) == l_undef) {
                         literal lit(i, true);
-                        validate_unit(lit);
+                        // validate_unit(lit);
                         IF_VERBOSE(2, verbose_stream() << "new unit " << lit << "\n");
                         s.assign_unit(lit);
                         ++m_stats.m_num_units;
@@ -266,7 +271,7 @@ namespace sat {
                     VERIFY(i != j);
                     literal u(i, false);
                     literal v(j, false);
-                    IF_VERBOSE(0,
+                    IF_VERBOSE(10,
                                verbose_stream() << u << " " << c << "\n";
                                verbose_stream() << v << ": ";
                                for (cut const& d : cuts[v.var()]) verbose_stream() << d << "\n";);
@@ -278,20 +283,19 @@ namespace sat {
                     new_eq = true;
                     break;
                 }
-                if (true || m_config.m_full) {
-                    cut nc(c);
-                    nc.negate();
-                    if (cut2id.find(&nc, j)) {
-                        VERIFY(i != j); // maybe possible with don't cares
-                        literal u(i, false);
-                        literal v(j, true);
-                        certify_equivalence(u, v, c);
-                        // validate_eq(u, v);
-                        add_eq(u, v);
-                        TRACE("aig_simplifier", tout << u << " == " << v << "\n";);
-                        new_eq = true;
-                        break;
-                    }
+
+                cut nc(c);
+                nc.negate();
+                if (cut2id.find(&nc, j)) {
+                    if (i == j) continue;
+                    literal u(i, false);
+                    literal v(j, true);
+                    certify_equivalence(u, v, c);
+                    // validate_eq(u, v);
+                    add_eq(u, v);
+                    TRACE("aig_simplifier", tout << u << " == " << v << "\n";);
+                    new_eq = true;
+                    break;
                 }
                 cut2id.insert(&c, i);                
             }
@@ -389,72 +393,122 @@ namespace sat {
         }                
     }
 
+
+    void aig_simplifier::add_dont_cares(vector<cut_set> const& cuts) {
+        if (m_config.m_enable_dont_cares) {
+            cuts2pairs(cuts);
+            pairs2dont_cares();
+            dont_cares2cuts(cuts);
+        }
+    }
+
     /**
      * collect pairs of variables that occur in cut sets.
      */
-    void aig_simplifier::collect_pairs(vector<cut_set> const& cuts) {
+    void aig_simplifier::cuts2pairs(vector<cut_set> const& cuts) {
+        svector<var_pair> dcs;
+        for (auto const& p : m_pairs) {
+            if (p.op != none) 
+                dcs.push_back(p);
+        }
         m_pairs.reset();
-        for (unsigned k = cuts.size(); k-- > 0; ) {
-            for (auto const& c : cuts[k]) {
+        for (auto const& cs : cuts) {
+            for (auto const& c : cs) {
                 for (unsigned i = c.size(); i-- > 0; ) {
                     for (unsigned j = i; j-- > 0; ) {
-                        m_pairs.insert(var_pair(c[i],c[j]));
+                        m_pairs.insert(var_pair(c[j],c[i]));
                     }
                 }
             }
         }
+        // don't lose previous don't cares
+        for (auto const& p : dcs) {
+            if (m_pairs.contains(p)) 
+                m_pairs.insert(p);
+        }
     }
     
     /**
      * compute masks for pairs.
      */
-    void aig_simplifier::add_masks_to_pairs() {
+    void aig_simplifier::pairs2dont_cares() {
         big b(s.rand());
         b.init(s, true);
         for (auto& p : m_pairs) {
+            if (p.op != none) continue;
             literal u(p.u, false), v(p.v, false);
             // u -> v, then u & ~v is impossible
             if (b.connected(u, v)) {
-                add_mask(u, ~v, p);
+                p.op = pn;
             }
             else if (b.connected(u, ~v)) {                
-                add_mask(u, v, p);
+                p.op = pp;
             }
             else if (b.connected(~u, v)) {
-                add_mask(~u, ~v, p);
+                p.op = nn;
             }
             else if (b.connected(~u, ~v)) {
-                add_mask(~u, v, p);
-            }
-            else {
-                memset(p.masks, 0xFF, var_pair::size());
+                p.op = np;
             }
         }
+        IF_VERBOSE(2, {
+                unsigned n = 0; for (auto const& p : m_pairs) if (p.op != none) ++n;
+                verbose_stream() << n << " / " << m_pairs.size() << " don't cares\n";
+            });
     }
 
-    /*
-     * compute masks for each possible occurrence of u, v within 2-6 elements.
-     * combinaions relative to u.sign(), v.sign() are impossible.
-     */ 
-    void aig_simplifier::add_mask(literal u, literal v, var_pair& p) {
-        unsigned offset = 0;
-        bool su = u.sign(), sv = v.sign();
-        for (unsigned k = 2; k <= 6; ++k) {
-            for (unsigned i = 0; i < k; ++i) {
-                for (unsigned j = i + 1; j < k; ++j) {
-                    // convert su, sv, k, i, j into a mask for 2^k bits.
-                    // for outputs 
-                    p.masks[offset++] = 0;
+    void aig_simplifier::dont_cares2cuts(vector<cut_set> const& cuts) {
+        struct rep { 
+            cut src, dst; unsigned v; 
+            rep(cut const& s, cut const& d, unsigned v):src(s), dst(d), v(v) {} 
+            rep():v(UINT_MAX) {}
+        };
+        vector<rep> to_replace;
+        cut d;
+        for (auto const& cs : cuts) {
+            for (auto const& c : cs) {
+                if (rewrite_cut(c, d)) {
+                    to_replace.push_back(rep(c, d, cs.var()));
                 }
             }
         }
+        for (auto const& p : to_replace) {
+            m_aig_cuts.replace(p.v, p.src, p.dst);
+        }
+        m_stats.m_num_dont_care_reductions += to_replace.size();
+    }
+
+    /*
+     * compute masks for position i, j and op-code p.op
+     */ 
+    uint64_t aig_simplifier::op2dont_care(unsigned i, unsigned j, var_pair const& p) {
+        SASSERT(i < j && j < 6);
+        if (p.op == none) return 0ull;
+        // first position of mask is offset into output bits contributed by i and j
+        bool i_is_0 = (p.op == np || p.op == nn);
+        bool j_is_0 = (p.op == pn || p.op == nn);
+        uint64_t first = (i_is_0 ? 0 : (1 << i)) + (j_is_0 ? 0 : (1 << j));
+        uint64_t inc = 1ull << (j + 1);
+        uint64_t r = 1ull << first; 
+        while (inc < 64ull) { r |= (r << inc); inc *= 2; }
+        return r;
     }
 
     /**
-     * apply obtained masks to cut sets.
+     * apply obtained dont_cares to cut sets.
      */
-    void aig_simplifier::apply_masks() {
-        
+    bool aig_simplifier::rewrite_cut(cut const& c, cut& d) {
+        bool init = false;
+        for (unsigned i = 0; i < c.size(); ++i) {
+            for (unsigned j = i + 1; j < c.size(); ++j) {
+                var_pair p(c[i], c[j]);
+                if (m_pairs.find(p, p) && p.op != none) {
+                    if (!init) { d = c; init = true; }
+                    d.set_table(d.m_table | op2dont_care(i, j, p));
+                }
+            }
+        }
+        return init && d.m_table != c.m_table;
     }
 
     void aig_simplifier::collect_statistics(statistics& st) const {
@@ -463,6 +517,7 @@ namespace sat {
         st.update("sat-aig.ands", m_stats.m_num_ands);
         st.update("sat-aig.ites", m_stats.m_num_ites);
         st.update("sat-aig.xors", m_stats.m_num_xors);
+        st.update("sat-aig.dc-reduce", m_stats.m_num_dont_care_reductions);
     }
 
     void aig_simplifier::validate_unit(literal lit) {
diff --git a/src/sat/sat_aig_simplifier.h b/src/sat/sat_aig_simplifier.h
index 0ace074b9..ba5d40561 100644
--- a/src/sat/sat_aig_simplifier.h
+++ b/src/sat/sat_aig_simplifier.h
@@ -27,13 +27,20 @@ namespace sat {
     public:
         struct stats {
             unsigned m_num_eqs, m_num_units, m_num_cuts, m_num_xors, m_num_ands, m_num_ites;
-            unsigned m_num_calls;
+            unsigned m_num_calls, m_num_dont_care_reductions;
             stats() { reset(); }
             void reset() { memset(this, 0, sizeof(*this)); }
         };
         struct config {
-            bool m_full;
-            config():m_full(false) {}
+            bool m_validate;
+            bool m_enable_units;
+            bool m_enable_dont_cares;
+            bool m_add_learned;
+            config():
+                m_validate(false), 
+                m_enable_units(false),
+                m_enable_dont_cares(false),
+                m_add_learned(true) {}
         };
     private:
         struct report;
@@ -60,14 +67,16 @@ namespace sat {
          * Apply the masks on cut sets so to allow detecting 
          * equivalences modulo implications.
          */
+
+        enum op_code { pp, pn, np, nn, none };
+
         struct var_pair {
             unsigned u, v;
-            uint64_t masks[35];
-            static unsigned size() { return sizeof(uint64_t)*35; }
-            var_pair(unsigned u, unsigned v): u(u), v(v) {
+            op_code op;
+            var_pair(unsigned _u, unsigned _v): u(_u), v(_v), op(none) {
                 if (u > v) std::swap(u, v);
             }
-            var_pair(): u(UINT_MAX), v(UINT_MAX) {}
+            var_pair(): u(UINT_MAX), v(UINT_MAX), op(none) {}
 
             struct hash {
                 unsigned operator()(var_pair const& p) const { 
@@ -82,10 +91,13 @@ namespace sat {
         };
         hashtable<var_pair, var_pair::hash, var_pair::eq> m_pairs;
         
-        void collect_pairs(vector<cut_set> const& cuts);
-        void add_mask(literal u, literal v, var_pair& p);
-        void add_masks_to_pairs();
-        void apply_masks();
+        void add_dont_cares(vector<cut_set> const& cuts);
+        void cuts2pairs(vector<cut_set> const& cuts);
+        void pairs2dont_cares();
+        void dont_cares2cuts(vector<cut_set> const& cuts);
+        bool rewrite_cut(cut const& c, cut& r);
+        uint64_t op2dont_care(unsigned i, unsigned j, var_pair const& p);
+
     public:
         aig_simplifier(solver& s);
         ~aig_simplifier();                
diff --git a/src/sat/sat_cutset.cpp b/src/sat/sat_cutset.cpp
index ca1f4690b..3b56b02ab 100644
--- a/src/sat/sat_cutset.cpp
+++ b/src/sat/sat_cutset.cpp
@@ -31,7 +31,7 @@ namespace sat {
        - pre-allocate fixed array instead of vector for cut_set to avoid overhead for memory allocation.
     */
     
-    bool cut_set::insert(on_update_t* on_add, on_update_t* on_del, cut const& c) {
+    bool cut_set::insert(on_update_t& on_add, on_update_t& on_del, cut const& c) {
         unsigned i = 0, j = 0, k = m_size;
         for (; i < k; ++i) {
             cut const& a = (*this)[i];
@@ -42,8 +42,11 @@ namespace sat {
                 std::swap(m_cuts[i--], m_cuts[--k]);
             }
         }
-        shrink(on_del, i);    
+        // for DRAT make sure to add new element before removing old cuts
+        // the new cut may need to be justified relative to the old cut
         push_back(on_add, c);
+        std::swap(m_cuts[i++], m_cuts[m_size-1]);
+        shrink(on_del, i);    
         return true;
     }
     
@@ -64,16 +67,16 @@ namespace sat {
     }
 
 
-    void cut_set::shrink(on_update_t* on_del, unsigned j) { 
-        if (m_var != UINT_MAX && on_del && *on_del) {
+    void cut_set::shrink(on_update_t& on_del, unsigned j) { 
+        if (m_var != UINT_MAX && on_del) {
             for (unsigned i = j; i < m_size; ++i) {
-                (*on_del)(m_var, m_cuts[i]);
+                on_del(m_var, m_cuts[i]);
             }
         }
         m_size = j; 
     }
 
-    void cut_set::push_back(on_update_t* on_add, cut const& c) {
+    void cut_set::push_back(on_update_t& on_add, cut const& c) {
         SASSERT(m_max_size > 0);
         if (m_size == m_max_size) {
             m_max_size *= 2;
@@ -81,10 +84,26 @@ namespace sat {
             memcpy(new_cuts, m_cuts, sizeof(cut)*m_size);
             m_cuts = new_cuts;
         }
-        if (m_var != UINT_MAX && on_add && *on_add) (*on_add)(m_var, c);
+        if (m_var != UINT_MAX && on_add) on_add(m_var, c);
         m_cuts[m_size++] = c; 
     }
 
+    void cut_set::replace(on_update_t& on_add, on_update_t& on_del, cut const& src, cut const& dst) {
+        SASSERT(src != dst);
+        insert(on_add, on_del, dst);
+        for (unsigned i = 0; i < size(); ++i) {
+            if (src == (*this)[i]) {
+                evict(on_del, i);
+                break;
+            }
+        }
+    }
+
+    void cut_set::evict(on_update_t& on_del, unsigned idx) {
+        if (m_var != UINT_MAX && on_del) on_del(m_var, m_cuts[idx]); 
+        m_cuts[idx] = m_cuts[--m_size]; 
+    }
+
     void cut_set::init(region& r, unsigned max_sz, unsigned v) { 
         m_var = v;
         m_max_size = max_sz;
diff --git a/src/sat/sat_cutset.h b/src/sat/sat_cutset.h
index ca496ed39..dc6f0c4b7 100644
--- a/src/sat/sat_cutset.h
+++ b/src/sat/sat_cutset.h
@@ -137,18 +137,27 @@ namespace sat {
 
         cut_set(): m_var(UINT_MAX), m_region(nullptr), m_size(0), m_max_size(0), m_cuts(nullptr) {}
         void init(region& r, unsigned max_sz, unsigned v);
-        bool insert(on_update_t* on_add, on_update_t* on_del, cut const& c);
+        bool insert(on_update_t& on_add, on_update_t& on_del, cut const& c);
         bool no_duplicates() const;
+        unsigned var() const { return m_var; }
         unsigned size() const { return m_size; }
         cut const * begin() const { return m_cuts; }
         cut const * end() const { return m_cuts + m_size; }
         cut const & back() { return m_cuts[m_size-1]; }
-        void push_back(on_update_t* on_add, cut const& c);
-        void reset(on_update_t* on_del) { shrink(on_del, 0); }
+        void push_back(on_update_t& on_add, cut const& c);
+        void reset(on_update_t& on_del) { shrink(on_del, 0); }
         cut const & operator[](unsigned idx) { return m_cuts[idx]; }
-        void shrink(on_update_t* on_del, unsigned j); 
-        void swap(cut_set& other) { std::swap(m_size, other.m_size); std::swap(m_cuts, other.m_cuts); std::swap(m_max_size, other.m_max_size); }
-        void evict(on_update_t* on_del, unsigned idx) { if (m_var != UINT_MAX && on_del && *on_del) (*on_del)(m_var, m_cuts[idx]); m_cuts[idx] = m_cuts[--m_size]; }
+        void shrink(on_update_t& on_del, unsigned j); 
+        void swap(cut_set& other) { 
+            std::swap(m_var, other.m_var);
+            std::swap(m_size, other.m_size); 
+            std::swap(m_max_size, other.m_max_size); 
+            std::swap(m_cuts, other.m_cuts); 
+        }
+        void evict(on_update_t& on_del, unsigned idx);
+
+        void replace(on_update_t& on_add, on_update_t& on_del, cut const& src, cut const& dst);
+
         std::ostream& display(std::ostream& out) const;
     };
 
diff --git a/src/util/symbol.cpp b/src/util/symbol.cpp
index febe79dbb..042d6292d 100644
--- a/src/util/symbol.cpp
+++ b/src/util/symbol.cpp
@@ -122,7 +122,7 @@ symbol::symbol(char const * d) {
 }
 
 symbol & symbol::operator=(char const * d) {
-    m_data = g_symbol_tables->get_str(d);
+    m_data = d ? g_symbol_tables->get_str(d) : nullptr;
     return *this;
 }