Polysat updates (#5444)

* Simplify adding lemmas

* Remove misleading constructor from tmp_assign.

The idea is that tmp_assign is only created on the stack and
short-lived.  Instead of having a convenience constructor that takes a
constraint_ref, it's clearer to have an explicit .get() at the call
site.

* Remove some log messages

* bugfix

* fix

* Add stub for conflict_core

* wip

* Add example by Clemens

src/math/polysat/conflict_core.h

@@ -0,0 +1,66 @@
+/*++
+Copyright (c) 2021 Microsoft Corporation
+
+Module Name:
+
+    polysat conflict
+
+Author:
+
+    Nikolaj Bjorner (nbjorner) 2021-03-19
+    Jakob Rath 2021-04-6
+
+--*/
+#pragma once
+#include "math/polysat/constraint.h"
+
+namespace polysat {
+
+    /// Represents a conflict as core (~negation of clause).
+    ///
+    /// TODO: can probably move some clause_builder functionality into this class.
+    class conflict_core {
+        // constraint_ref_vector m_constraints;
+        vector<constraint_literal> m_constraints;
+        bool m_needs_model = true;  ///< True iff the conflict depends on the current variable assignment. (If so, additional constraints must be added to the final learned clause.)
+
+    public:
+        vector<constraint_literal> const& constraints() const {
+            return m_constraints;
+        }
+
+        bool empty() const {
+            return m_constraints.empty();
+        }
+
+        void reset() {
+            m_constraints.reset();
+            m_needs_model = true;
+        }
+
+        // for bailing out with a conflict at the base level
+        void set(std::nullptr_t) {
+            SASSERT(empty());
+            m_constraints.push_back({});
+            m_needs_model = false;
+        }
+
+        // void set(
+        
+        // TODO: set core from conflicting units
+        // TODO: set clause
+
+        // TODO: use iterator instead (this method is needed for marking so duplicates don't necessarily have to be skipped)
+        unsigned_vector vars(constraint_manager const& cm) const {
+            unsigned_vector vars;
+            for (auto const& c : m_constraints)
+                vars.append(c->vars());
+            return vars;
+        }
+
+        std::ostream& display(std::ostream& out) const;
+    };
+
+    inline std::ostream& operator<<(std::ostream& out, conflict_core const& c) { return c.display(out); }
+
+}