feat(kernel/definition): default constructor for definitions

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

src/kernel/definition.cpp

@@ -59,6 +59,9 @@ struct definition::cell {
     }
 };
 
+definition g_dummy = mk_axiom(name(), param_names(), level_cnstrs(), expr());
+
+definition::definition():definition(g_dummy) {}
 definition::definition(cell * ptr):m_ptr(ptr) {}
 definition::definition(definition const & s):m_ptr(s.m_ptr) { if (m_ptr) m_ptr->inc_ref(); }
 definition::definition(definition && s):m_ptr(s.m_ptr) { s.m_ptr = nullptr; }

src/kernel/definition.h

@@ -20,6 +20,15 @@ class definition {
     explicit definition(cell * ptr);
     friend class cell;
 public:
+    /**
+       \brief The default constructor creates a reference to a "dummy"
+       definition.  The actual "dummy" definition is not relevant, and
+       no procedure should rely on the kind of definition used.
+
+       We have a default constructor because some collections only work
+       with types that have a default constructor.
+    */
+    definition();
     definition(definition const & s);
     definition(definition && s);
     ~definition();