change queries to take function names instead of arbitrary predicates. This allows to bypass issues with having arbitrary query expressions compiled in arbitrary ways to auxiliary predicates where names of bound variables are reshuffled. See also Stackoverflow http://stackoverflow.com/questions/34693719/bug-in-z3-datalog

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/muz/base/dl_context.cpp

@@ -891,6 +891,7 @@ namespace datalog {
     }
 
     lbool context::rel_query(unsigned num_rels, func_decl * const* rels) {        
+        m_last_answer = 0;
         ensure_engine();
         return m_engine->query(num_rels, rels);
     }