From bcf0ee77096fd70d3cd60da2097c2562ca6bf249 Mon Sep 17 00:00:00 2001
From: Nikolaj Bjorner <nbjorner@microsoft.com>
Date: Thu, 29 Jun 2017 18:53:58 -0700
Subject: [PATCH] n/a

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>
---
 src/sat/ba_solver.cpp   | 255 +++++++++++++++++++++++++++++++++-------
 src/sat/ba_solver.h     |  29 ++++-
 src/sat/sat_extension.h |   1 +
 src/sat/sat_lookahead.h |   1 +
 src/sat/sat_solver.cpp  |  21 ++--
 src/sat/sat_watched.cpp |   3 +-
 src/sat/sat_watched.h   |   2 +-
 7 files changed, 252 insertions(+), 60 deletions(-)

diff --git a/src/sat/ba_solver.cpp b/src/sat/ba_solver.cpp
index 7d9e16df0..22c77f164 100644
--- a/src/sat/ba_solver.cpp
+++ b/src/sat/ba_solver.cpp
@@ -69,6 +69,14 @@ namespace sat {
         SASSERT(m_size >= m_k && m_k > 0);
     }
 
+    bool ba_solver::card::is_watching(literal l) const {
+        unsigned sz = std::min(k() + 1, size());
+        for (unsigned i = 0; i < sz; ++i) {
+            if ((*this)[i] == l) return true;
+        }
+        return false;
+    }
+
     std::ostream& operator<<(std::ostream& out, ba_solver::constraint const& cnstr) {
         if (cnstr.lit() != null_literal) out << cnstr.lit() << " == ";
         switch (cnstr.tag()) {
@@ -135,6 +143,14 @@ namespace sat {
         m_k = w - m_k + 1;
         SASSERT(w >= m_k && m_k > 0);
     }
+
+    bool ba_solver::pb::is_watching(literal l) const {
+        for (unsigned i = 0; i < m_num_watch; ++i) {
+            if ((*this)[i].second == l) return true;
+        }
+        return false;
+    }
+
     
     ba_solver::xor::xor(literal lit, literal_vector const& lits):
     constraint(xor_t, lit, lits.size(), get_obj_size(lits.size())) {
@@ -143,6 +159,12 @@ namespace sat {
         }
     }
 
+    bool ba_solver::xor::is_watching(literal l) const {
+        return 
+            l == (*this)[0] || l == (*this)[1] ||
+            ~l == (*this)[0] || ~l == (*this)[1];            
+    }
+
     void ba_solver::init_watch(card& c, bool is_true) {
         clear_watch(c);
         if (c.lit() != null_literal && c.lit().sign() == is_true) {
@@ -211,14 +233,6 @@ namespace sat {
         }
     }
 
-    void ba_solver::unwatch_literal(literal lit, constraint& c) {
-        get_wlist(~lit).erase(watched(c.index()));
-    }
-
-    void ba_solver::watch_literal(literal lit, constraint& c) {
-        get_wlist(~lit).push_back(watched(c.index()));
-    }
-
     void ba_solver::set_conflict(constraint& c, literal lit) {
         m_stats.m_num_conflicts++;
         TRACE("sat", display(tout, c, true); );
@@ -510,7 +524,6 @@ namespace sat {
     }
 
     void ba_solver::simplify(pb& p) {
-        s().pop_to_base_level();
         if (p.lit() != null_literal && value(p.lit()) == l_false) {
             TRACE("sat", tout << "pb: flip sign " << p << "\n";);
             return;
@@ -629,6 +642,8 @@ namespace sat {
     void ba_solver::clear_watch(xor& x) {
         unwatch_literal(x[0], x);
         unwatch_literal(x[1], x);         
+        unwatch_literal(~x[0], x);
+        unwatch_literal(~x[1], x);         
     }
 
     bool ba_solver::parity(xor const& x, unsigned offset) const {
@@ -678,6 +693,8 @@ namespace sat {
             SASSERT(j == 2);
             watch_literal(x[0], x);
             watch_literal(x[1], x);
+            watch_literal(~x[0], x);
+            watch_literal(~x[1], x);
             break;
         }
     }
@@ -1157,6 +1174,14 @@ namespace sat {
             s().set_external(lit.var());
             get_wlist(lit).push_back(c->index());
             get_wlist(~lit).push_back(c->index());
+            if (!validate_watched_constraint(*c)) {
+                std::cout << "wrong: " << *c << "\n";
+            }
+        }        
+        if (lit.var() == 102770) {
+            display(std::cout, *c, true);
+            display_watch_list(std::cout, s().m_cls_allocator, get_wlist(lit)) << "\n";
+            display_watch_list(std::cout, s().m_cls_allocator, get_wlist(~lit)) << "\n";
         }
     }
 
@@ -1208,6 +1233,7 @@ namespace sat {
         if (c.lit() != null_literal && l.var() == c.lit().var()) {
             init_watch(c, !l.sign());
             keep = true;
+            if (!inconsistent()) validate_watched_constraint(c);
         }
         else if (c.lit() != null_literal && value(c.lit()) != l_true) {
             keep = false;
@@ -1215,6 +1241,7 @@ namespace sat {
         else {
             keep = l_undef != add_assign(c, ~l);
         }
+        std::cout << c.lit() << " " << l << " " << keep << "\n";
     }
 
 
@@ -1444,10 +1471,21 @@ namespace sat {
         }
     }
 
+    // ----------------------------
+    // constraint generic methods
+
     void ba_solver::get_antecedents(literal l, ext_justification_idx idx, literal_vector & r) {
         get_antecedents(l, index2constraint(idx), r);
     }
     
+    void ba_solver::unwatch_literal(literal lit, constraint& c) {
+        get_wlist(~lit).erase(watched(c.index()));
+    }
+
+    void ba_solver::watch_literal(literal lit, constraint& c) {
+        get_wlist(~lit).push_back(watched(c.index()));
+    }
+
     void ba_solver::get_antecedents(literal l, constraint const& c, literal_vector& r) {
         switch (c.tag()) {
         case card_t: get_antecedents(l, c.to_card(), r); break;
@@ -1457,7 +1495,38 @@ namespace sat {
         }
     }
 
+    void ba_solver::nullify_tracking_literal(constraint& c) {
+        if (c.lit() != null_literal) {
+            get_wlist(c.lit()).erase(watched(c.index()));
+            get_wlist(~c.lit()).erase(watched(c.index()));
+            c.nullify_literal();
+        }
+    }
+
+    void ba_solver::remove_constraint(constraint& c) {
+        nullify_tracking_literal(c);
+        switch (c.tag()) {
+        case card_t:
+            clear_watch(c.to_card());
+            break;
+        case pb_t:
+            clear_watch(c.to_pb());
+            break;
+        case xor_t:
+            clear_watch(c.to_xor());
+            break;
+        default:
+            UNREACHABLE();
+        }
+        c.remove();
+        m_constraint_removed = true;
+    }
+
+    // --------------------------------
+    // validation
+
     bool ba_solver::validate_unit_propagation(constraint const& c, literal l) const {
+        return true;
         switch (c.tag()) {
         case card_t:  return validate_unit_propagation(c.to_card(), l); 
         case pb_t: return validate_unit_propagation(c.to_pb(), l);
@@ -1468,6 +1537,7 @@ namespace sat {
     }
 
     bool ba_solver::validate_conflict(constraint const& c) const {
+        return true;
         switch (c.tag()) {
         case card_t:  return validate_conflict(c.to_card()); 
         case pb_t: return validate_conflict(c.to_pb());
@@ -1477,6 +1547,133 @@ namespace sat {
         return false;
     }
 
+    bool ba_solver::is_true(constraint const& c) const {
+        lbool v1 = c.lit() == null_literal ? l_true : value(c.lit());
+        if (v1 == l_undef) return false;
+        switch (c.tag()) {
+        case card_t:  return v1 == value(c.to_card()); 
+        case pb_t: return v1 == value(c.to_pb());
+        case xor_t: return v1 == value(c.to_xor());
+        default: UNREACHABLE(); break;
+        }
+        return false;        
+    }
+
+    lbool ba_solver::value(card const& c) const {
+        unsigned trues = 0, undefs = 0;
+        for (literal l : c) {
+            switch (value(l)) {
+            case l_true: trues++; break;
+            case l_undef: undefs++; break;
+            default: break;
+            }
+        }
+        if (trues + undefs < c.k()) return l_false;
+        if (trues >= c.k()) return l_true;
+        return l_undef;        
+    }
+
+    lbool ba_solver::value(pb const& p) const {
+        unsigned trues = 0, undefs = 0;
+        for (wliteral wl : p) {
+            switch (value(wl.second)) {
+            case l_true: trues += wl.first; break;
+            case l_undef: undefs += wl.first; break;
+            default: break;
+            }
+        }
+        if (trues + undefs < p.k()) return l_false;
+        if (trues >= p.k()) return l_true;
+        return l_undef;        
+    }
+
+    lbool ba_solver::value(xor const& x) const {
+        bool odd = false;
+        
+        for (auto l : x) {
+            switch (value(l)) {
+            case l_true: odd = !odd; break;
+            case l_false: break;
+            default: return l_undef;
+            }
+        }
+        return odd ? l_true : l_false;
+    }
+
+    void ba_solver::validate() {
+        if (validate_watch_literals()) {
+            for (constraint* c : m_constraints) {
+                if (!validate_watched_constraint(*c)) break;
+            }
+        }
+    }
+
+    bool ba_solver::validate_watch_literals() const {
+        for (unsigned v = 0; v < s().num_vars(); ++v) {
+            literal lit(v, false);
+            if (lvl(lit) == 0) continue;
+            if (!validate_watch_literal(lit)) return false;
+            if (!validate_watch_literal(~lit)) return false;
+        }        
+        return true;
+    }
+
+    bool ba_solver::validate_watch_literal(literal lit) const {
+        if (lvl(lit) == 0) return true;
+        for (auto const & w : get_wlist(lit)) {
+            if (w.get_kind() == watched::EXT_CONSTRAINT) {
+                constraint const& c = index2constraint(w.get_ext_constraint_idx());
+                if (!c.is_watching(~lit)) {
+                    std::cout << lit << " " << lvl(lit) << " is not watched in " << c << "\n";
+                    display(std::cout, c, true);
+                    UNREACHABLE();
+                    return false;
+                }
+            }
+        }
+        return true;
+    }
+
+    bool ba_solver::validate_watched_constraint(constraint const& c) const {
+        if (c.lit() != null_literal && value(c.lit()) != l_true) return true;
+        if (c.lit() != null_literal && lvl(c.lit()) != 0) {
+            if (!is_watching(c.lit(), c) || !is_watching(~c.lit(), c)) {
+                std::cout << "Definition literal is not watched " << c.lit() << " " << c << "\n";
+                display_watch_list(std::cout, s().m_cls_allocator, get_wlist(c.lit())) << "\n";
+                display_watch_list(std::cout, s().m_cls_allocator, get_wlist(~c.lit())) << "\n";
+                return false;
+            }
+        }
+        if (is_true(c)) {
+            return true;
+        }
+        literal_vector lits(c.literals());
+        for (literal l : lits) {
+            if (lvl(l) == 0) continue;
+            bool found = is_watching(l, c);
+            if (found != c.is_watching(l)) {
+                std::cout << "Discrepancy of watched literal: " << l << ": " << c.index() << " " << c << (found?" is watched, but shouldn't be":" not watched, but should be") << "\n";
+                display_watch_list(std::cout << l << ": ", s().m_cls_allocator, get_wlist(l)) << "\n";
+                display_watch_list(std::cout << ~l << ": ", s().m_cls_allocator, get_wlist(~l)) << "\n";
+                std::cout << "value: " << value(l) << " level: " << lvl(l) << "\n";
+                display(std::cout, c, true);
+                if (c.lit() != null_literal) std::cout << value(c.lit()) << "\n";
+                UNREACHABLE();
+                exit(1);
+                return false;
+            }
+        }
+        return true;
+    }
+    
+    bool ba_solver::is_watching(literal lit, constraint const& c) const {
+        for (auto w : get_wlist(~lit)) {
+            if (w.get_kind() == watched::EXT_CONSTRAINT && w.get_ext_constraint_idx() == c.index()) 
+                return true;
+        }
+        return false;
+    }
+
     /**
        \brief Lex on (glue, size)
     */
@@ -1507,33 +1704,6 @@ namespace sat {
 
     }
 
-    void ba_solver::nullify_tracking_literal(constraint& c) {
-        if (c.lit() != null_literal) {
-            get_wlist(c.lit()).erase(watched(c.index()));
-            get_wlist(~c.lit()).erase(watched(c.index()));
-            c.nullify_literal();
-        }
-    }
-
-    void ba_solver::remove_constraint(constraint& c) {
-        nullify_tracking_literal(c);
-        switch (c.tag()) {
-        case card_t:
-            clear_watch(c.to_card());
-            break;
-        case pb_t:
-            clear_watch(c.to_pb());
-            break;
-        case xor_t:
-            clear_watch(c.to_xor());
-            break;
-        default:
-            UNREACHABLE();
-        }
-        c.remove();
-        m_constraint_removed = true;
-    }
-
     void ba_solver::simplify(card& c) {
         SASSERT(c.lit() == null_literal || value(c.lit()) != l_false);
         if (c.lit() != null_literal && value(c.lit()) == l_false) {
@@ -1722,7 +1892,7 @@ namespace sat {
 
     void ba_solver::simplify() {
         return;
-        if (!s().at_base_lvl()) s().pop_to_base_level();
+        SASSERT(s().at_base_lvl());
         unsigned trail_sz;
         do {
             m_simplify_change = false;
@@ -1760,6 +1930,10 @@ namespace sat {
 
     void ba_solver::flush_roots() {
         if (m_roots.empty()) return;
+
+        std::cout << "pre\n";
+        validate();
+
         m_visited.resize(s().num_vars()*2, false);
         m_constraint_removed = false;
         for (constraint* c : m_constraints) {
@@ -1778,6 +1952,8 @@ namespace sat {
             }                
         }
         cleanup_constraints();
+        std::cout << "post\n";
+        validate();
         // display(std::cout << "flush roots\n");
     }
 
@@ -1907,7 +2083,7 @@ namespace sat {
     }
 
     void ba_solver::recompile(pb& p) {
-        // IF_VERBOSE(0, verbose_stream() << "re: " << p << "\n";);
+        IF_VERBOSE(0, verbose_stream() << "re: " << p << "\n";);
         m_weights.resize(2*s().num_vars(), 0);
         for (wliteral wl : p) {
             m_weights[wl.second.index()] += wl.first;
@@ -1965,11 +2141,10 @@ namespace sat {
         p.update_k(k);
         p.update_max_sum();
 
-
         literal root = null_literal;
         if (p.lit() != null_literal) root = m_roots[p.lit().index()];
 
-        // IF_VERBOSE(0, verbose_stream() << "new: " << p << "\n";);
+        IF_VERBOSE(0, verbose_stream() << "new: " << p << "\n";);
 
         // std::cout << "simplified " << p << "\n";
         if (p.lit() != root) {
diff --git a/src/sat/ba_solver.h b/src/sat/ba_solver.h
index ca6b0d093..2c2a5d914 100644
--- a/src/sat/ba_solver.h
+++ b/src/sat/ba_solver.h
@@ -74,6 +74,7 @@ namespace sat {
             void nullify_literal() { m_lit = null_literal; }
             unsigned glue() const { return m_glue; }
             void set_glue(unsigned g) { m_glue = g; }
+            
 
             size_t obj_size() const { return m_obj_size; }
             card& to_card();
@@ -85,6 +86,9 @@ namespace sat {
             bool is_card() const { return m_tag == card_t; }
             bool is_pb() const { return m_tag == pb_t; }
             bool is_xor() const { return m_tag == xor_t; }
+            
+            virtual bool is_watching(literal l) const { return false; };
+            virtual literal_vector literals() const { return literal_vector(); }
         };
 
         friend std::ostream& operator<<(std::ostream& out, constraint const& c);
@@ -103,7 +107,8 @@ namespace sat {
             void swap(unsigned i, unsigned j) { std::swap(m_lits[i], m_lits[j]); }
             void negate();                     
             void update_k(unsigned k) { m_k = k; }
-            literal_vector literals() const { return literal_vector(m_size, m_lits); }            
+            virtual literal_vector literals() const { return literal_vector(m_size, m_lits); }            
+            virtual bool is_watching(literal l) const;
         };
 
         
@@ -122,7 +127,7 @@ namespace sat {
             wliteral operator[](unsigned i) const { return m_wlits[i]; }
             wliteral& operator[](unsigned i) { return m_wlits[i]; }
             wliteral const* begin() const { return m_wlits; }
-            wliteral const* end() const { return static_cast<wliteral const*>(m_wlits) + m_size; }
+            wliteral const* end() const { return begin() + m_size; }
 
             unsigned k() const { return m_k; }
             unsigned slack() const { return m_slack; }
@@ -134,7 +139,8 @@ namespace sat {
             void negate();                       
             void update_k(unsigned k) { m_k = k; }
             void update_max_sum();
-            literal_vector literals() const { literal_vector lits; for (auto wl : *this) lits.push_back(wl.second); return lits; }
+            virtual literal_vector literals() const { literal_vector lits; for (auto wl : *this) lits.push_back(wl.second); return lits; }
+            virtual bool is_watching(literal l) const;
         };
 
         class xor : public constraint {
@@ -144,9 +150,11 @@ namespace sat {
             xor(literal lit, literal_vector const& lits);
             literal operator[](unsigned i) const { return m_lits[i]; }
             literal const* begin() const { return m_lits; }
-            literal const* end() const { return static_cast<literal const*>(m_lits) + m_size; }
+            literal const* end() const { return begin() + m_size; }
             void swap(unsigned i, unsigned j) { std::swap(m_lits[i], m_lits[j]); }
             void negate() { m_lits[0].neg(); }
+            virtual bool is_watching(literal l) const;
+            virtual literal_vector literals() const { return literal_vector(size(), begin()); }
         };
 
 
@@ -244,6 +252,9 @@ namespace sat {
         void get_antecedents(literal l, constraint const& c, literal_vector & r);
         bool validate_conflict(constraint const& c) const;
         bool validate_unit_propagation(constraint const& c, literal alit) const;
+        void attach_constraint(constraint const& c);
+        void detach_constraint(constraint const& c);
+        bool is_true(constraint const& c) const;
 
         // cardinality
         void init_watch(card& c, bool is_true);
@@ -256,6 +267,7 @@ namespace sat {
         void unit_propagation_simplification(literal lit, literal_vector const& lits);
         void flush_roots(card& c);
         void recompile(card& c);
+        lbool value(card const& c) const;
 
         // xor specific functionality
         void clear_watch(xor& x);
@@ -266,6 +278,7 @@ namespace sat {
         void get_antecedents(literal l, xor const& x, literal_vector & r);
         void simplify(xor& x);
         void flush_roots(xor& x);
+        lbool value(xor const& x) const;
         
         // pb functionality
         unsigned m_a_max;
@@ -279,6 +292,7 @@ namespace sat {
         bool is_cardinality(pb const& p);
         void flush_roots(pb& p);
         void recompile(pb& p);
+        lbool value(pb const& p) const;
 
         // access solver
         inline lbool value(literal lit) const { return m_lookahead ? m_lookahead->value(lit) : m_solver->value(lit); }
@@ -286,6 +300,7 @@ namespace sat {
         inline unsigned lvl(bool_var v) const { return m_solver->lvl(v); }
         inline bool inconsistent() const { return m_lookahead ? m_lookahead->inconsistent() : m_solver->inconsistent(); }
         inline watch_list& get_wlist(literal l) { return m_lookahead ? m_lookahead->get_wlist(l) : m_solver->get_wlist(l); }
+        inline watch_list const& get_wlist(literal l) const { return m_lookahead ? m_lookahead->get_wlist(l) : m_solver->get_wlist(l); }
         inline void assign(literal l, justification j) { if (m_lookahead) m_lookahead->assign(l); else m_solver->assign(l, j); }
         inline void set_conflict(justification j, literal l) { if (m_lookahead) m_lookahead->set_conflict(); else m_solver->set_conflict(j, l); }
         inline config const& get_config() const { return m_solver->get_config(); }
@@ -312,6 +327,10 @@ namespace sat {
         bool validate_unit_propagation(pb const& p, literal alit) const;
         bool validate_unit_propagation(xor const& x, literal alit) const;
         bool validate_conflict(literal_vector const& lits, ineq& p);
+        bool validate_watch_literals() const;
+        bool validate_watch_literal(literal lit) const;
+        bool validate_watched_constraint(constraint const& c) const;
+        bool is_watching(literal lit, constraint const& c) const;
 
         ineq m_A, m_B, m_C;
         void active2pb(ineq& p);
@@ -358,6 +377,8 @@ namespace sat {
 
         ptr_vector<constraint> const & constraints() const { return m_constraints; }
 
+        virtual void validate();
+
     };
 
 };
diff --git a/src/sat/sat_extension.h b/src/sat/sat_extension.h
index 642171610..f984091e3 100644
--- a/src/sat/sat_extension.h
+++ b/src/sat/sat_extension.h
@@ -53,6 +53,7 @@ namespace sat {
         virtual extension* copy(solver* s) = 0;       
         virtual void find_mutexes(literal_vector& lits, vector<literal_vector> & mutexes) = 0;
         virtual void gc() = 0;
+        virtual void validate() = 0;
     };
 
 };
diff --git a/src/sat/sat_lookahead.h b/src/sat/sat_lookahead.h
index 43c2612b8..067a95c55 100644
--- a/src/sat/sat_lookahead.h
+++ b/src/sat/sat_lookahead.h
@@ -372,6 +372,7 @@ namespace sat {
         void attach_ternary(ternary const& t);
         void attach_ternary(literal l1, literal l2, literal l3);
         watch_list& get_wlist(literal l) { return m_watches[l.index()]; }
+        watch_list const& get_wlist(literal l) const { return m_watches[l.index()]; }
 
         // ------------------------------------
         // initialization
diff --git a/src/sat/sat_solver.cpp b/src/sat/sat_solver.cpp
index 7a6955ef8..dfa85569c 100644
--- a/src/sat/sat_solver.cpp
+++ b/src/sat/sat_solver.cpp
@@ -1391,6 +1391,7 @@ namespace sat {
 
     */
     void solver::simplify_problem() {
+        if (m_ext) m_ext->validate();
         if (m_conflicts_since_init < m_next_simplify) {
             return;
         }
@@ -1403,12 +1404,15 @@ namespace sat {
         SASSERT(at_base_lvl());
 
         m_cleaner();
+        if (m_ext) m_ext->validate();
         CASSERT("sat_simplify_bug", check_invariant());
 
         m_scc();
+        if (m_ext) m_ext->validate();
         CASSERT("sat_simplify_bug", check_invariant());
 
         m_simplifier(false);
+        if (m_ext) m_ext->validate();
         CASSERT("sat_simplify_bug", check_invariant());
         CASSERT("sat_missed_prop", check_missed_propagation());
 
@@ -1417,6 +1421,7 @@ namespace sat {
             CASSERT("sat_missed_prop", check_missed_propagation());
             CASSERT("sat_simplify_bug", check_invariant());
         }
+        if (m_ext) m_ext->validate();
 
         if (m_config.m_lookahead_simplify) {
             {
@@ -1435,10 +1440,12 @@ namespace sat {
         CASSERT("sat_simplify_bug", check_invariant());
 
         m_probing();
+        if (m_ext) m_ext->validate();
         CASSERT("sat_missed_prop", check_missed_propagation());
         CASSERT("sat_simplify_bug", check_invariant());
 
         m_asymm_branch();
+        if (m_ext) m_ext->validate();
         CASSERT("sat_missed_prop", check_missed_propagation());
         CASSERT("sat_simplify_bug", check_invariant());
 
@@ -1460,21 +1467,7 @@ namespace sat {
                 m_next_simplify = m_conflicts_since_init + m_config.m_simplify_max;
         }
 
-#if 0
-        static unsigned file_no = 0;
-        #pragma omp critical (print_sat)
-        {
-            ++file_no;
-            std::ostringstream ostrm;
-            ostrm << "s" << file_no << ".txt";
-            std::ofstream ous(ostrm.str());
-            display(ous);
-        }
-#endif
-
         if (m_par) m_par->set_phase(*this);
-
-
     }
 
     bool solver::set_root(literal l, literal r) {
diff --git a/src/sat/sat_watched.cpp b/src/sat/sat_watched.cpp
index 1b294351f..d1642d50f 100644
--- a/src/sat/sat_watched.cpp
+++ b/src/sat/sat_watched.cpp
@@ -39,7 +39,7 @@ namespace sat {
         return false;                                           
     }
 
-    void display_watch_list(std::ostream & out, clause_allocator const & ca, watch_list const & wlist) {
+    std::ostream& display_watch_list(std::ostream & out, clause_allocator const & ca, watch_list const & wlist) {
         watch_list::const_iterator it  = wlist.begin();
         watch_list::const_iterator end = wlist.end();
         for (bool first = true; it != end; ++it) {
@@ -66,6 +66,7 @@ namespace sat {
                 UNREACHABLE();
             }
         }
+        return out;
     }
 
 };
diff --git a/src/sat/sat_watched.h b/src/sat/sat_watched.h
index b9a0962e9..fa7008818 100644
--- a/src/sat/sat_watched.h
+++ b/src/sat/sat_watched.h
@@ -130,7 +130,7 @@ namespace sat {
     inline void erase_ternary_watch(watch_list & wlist, literal l1, literal l2) { wlist.erase(watched(l1, l2)); }
 
     class clause_allocator;
-    void display_watch_list(std::ostream & out, clause_allocator const & ca, watch_list const & wlist);
+    std::ostream& display_watch_list(std::ostream & out, clause_allocator const & ca, watch_list const & wlist);
 };
 
 #endif