fix: Evaluation

src/prolog/builtins/unification.kt

@@ -0,0 +1,20 @@
+package prolog.builtins
+
+import prolog.components.terms.Atom
+import prolog.components.terms.Structure
+import prolog.components.terms.Term
+import prolog.components.terms.Variable
+
+/**
+ * True if Term1 is equivalent to Term2. A variable is only identical to a sharing variable.
+ */
+fun equivalent(term1: Term, term2: Term): Boolean {
+    return when {
+        term1 is Variable && term2 is Variable -> term1 == term2
+        term1 is Variable -> term1.alias().isPresent && equivalent(term1.alias().get(), term2)
+        term2 is Variable -> term2.alias().isPresent && equivalent(term2.alias().get(), term1)
+        term1 is Atom && term2 is Atom -> term1.compareTo(term2) == 0
+        term1 is Structure && term2 is Structure -> term1.compareTo(term2) == 0
+        else -> false
+    }
+}