Checkpoint

src/prolog/ast/terms/Structure.kt

@@ -1,7 +1,7 @@
 package prolog.ast.terms
 
 import prolog.ast.logic.Resolvent
-import prolog.builtins.equivalent
+import prolog.logic.equivalent
 import prolog.logic.Substituted
 import prolog.logic.unifyLazy
 
@@ -9,13 +9,19 @@ typealias Argument = Term
 
 typealias CompoundTerm = Structure
 
-open class Structure(val name: Atom, val arguments: List<Argument>) : Goal(), Head, Body, Resolvent {
+open class Structure(val name: Atom, var arguments: List<Argument>) : Goal(), Head, Body, Resolvent {
     override val functor: Functor = "${name.name}/${arguments.size}"
 
     override fun solve(goal: Goal, subs: Substituted): Sequence<Substituted> {
         return unifyLazy(goal, this, subs)
     }
 
+    // A structure does not need to be evaluated, so return an empty sequence.
+    override fun evaluate(subs: Substituted): Pair<Term, Substituted> = Pair(this, emptyMap())
+
+    /**
+     * See also [SWI Prolog Standard Order of Terms](https://www.swi-prolog.org/pldoc/man?section=standardorder)
+     */
     override fun compareTo(other: Term): Int {
         when (other) {
             is Structure -> {