Switch to representing propositional constant as Constant.

Make the fact format more consistent by representing propositional
constants and constant terms using the same Predicate, same as how
function terms and function atoms use the same Predicate.

src/renopro/asp/tests/reify/prop_fact.lp

@@ -4,7 +4,7 @@
 
 program("base",constant_tuple(0),statement_tuple(1)).
 statement_tuple(1,0,rule(2)).
-rule(2,literal(3),literal_tuple(6)).
+rule(2,literal(3),literal_tuple(5)).
 % facts representing head literal a.
-literal(3,"pos",function(4)).
-function(4,a,term_tuple(5)).
+literal(3,"pos",constant(4)).
+constant(4,a).

src/renopro/asp/tests/reify/prop_normal_rule.lp

@@ -4,15 +4,15 @@
 
 program("base",constant_tuple(0),statement_tuple(1)).
 statement_tuple(1,0,rule(2)).
-rule(2,literal(3),literal_tuple(6)).
+rule(2,literal(3),literal_tuple(5)).
 % facts representing head literal a.
-literal(3,"pos",function(4)).
-function(4,a,term_tuple(5)).
+literal(3,"pos",constant(4)).
+constant(4,a).
 % facts representing body literal b.
-literal_tuple(6,0,literal(7)).
-literal(7,"pos",function(8)).
-function(8,b,term_tuple(9)).
+literal_tuple(5,0,literal(6)).
+literal(6,"pos",constant(7)).
+constant(7,b).
 % facts representing body literal not c.
-literal_tuple(6,1,literal(10)).
-literal(10,"not",function(11)).
-function(11,c,term_tuple(12)).
+literal_tuple(5,1,literal(8)).
+literal(8,"not",constant(9)).
+constant(9,c).

src/renopro/clorm_predicates.py

@@ -164,6 +164,9 @@ class Binary_Operation(Predicate):
 
 Term.fields.append(Binary_Operation1.Field)
 
+# an atom may be a predicate (Function) or propositional constant (Constant)
+Atom = combine_fields([Function1.Field, Constant1.Field], name="Atom")
+
 
 class Sign(str, enum.Enum):
     DoubleNegation = "not not"
@@ -189,7 +192,7 @@ class Literal(Predicate):
     sig = define_enum_field(parent_field=StringField,
                             enum_class=Sign,
                             name="SignField")
-    atom = Function1.Field
+    atom = Atom
 
 
 Literal1 = make_id_predicate(Literal)

src/renopro/reify.py

@@ -192,6 +192,22 @@ def _reify_symbolic_atom(self, node):
 
     @_reify_ast.register(ASTType.Function)
     def _reify_function(self, node):
+        """Reify an ast node with node.ast_type of ASTType.Function.
+
+        Note that clingo's ast also represents propositional constants
+        as nodes with node.type of ASTType.Function and an empty
+        node.arguments list; thus some additional care must be taken
+        to create the correct clorm predicate.
+
+        """
+        if len(node.arguments) == 0:
+            constant1 = preds.Constant1()
+            constant = preds.Constant(
+                id=constant1.id,
+                name=node.name
+            )
+            self._reified.add(constant)
+            return constant1
         function1 = preds.Function1()
         function = preds.Function(
             id=function1.id,