Checkpoint

src/prolog/ast/logic/Clause.kt

@@ -37,8 +37,8 @@ abstract class Clause(private val head: Head, private val body: Body) : Resolven
 
     override fun toString(): String {
         return when {
-            body == True() -> head.toString()
-            else           -> "$head :- $body"
+            body is True -> head.toString()
+            else         -> "$head :- $body"
         }
     }
 }

src/prolog/ast/logic/Fact.kt

@@ -3,4 +3,4 @@ package prolog.ast.logic
 import prolog.ast.terms.Head
 import prolog.builtins.True
 
-class Fact(head: Head) : Clause(head, True())
+class Fact(head: Head) : Clause(head, True)

src/prolog/ast/logic/LogicOperand.kt

@@ -0,0 +1,5 @@
+package prolog.ast.logic
+
+import prolog.ast.terms.Operand
+
+abstract class LogicOperand : Operand, Provable

src/prolog/ast/logic/LogicOperator.kt

@@ -0,0 +1,13 @@
+package prolog.ast.logic
+
+import prolog.ast.terms.Atom
+import prolog.ast.terms.Operator
+import prolog.logic.Substituted
+
+abstract class LogicOperator(
+    symbol: Atom,
+    leftOperand: LogicOperand? = null,
+    rightOperand: LogicOperand
+) : Operator(symbol, leftOperand, rightOperand), Provable {
+    abstract override fun prove(subs: Substituted): Sequence<Substituted>
+}