refactor: Rework

src/prolog/Substitution.kt

@@ -0,0 +1,11 @@
+package prolog
+
+import prolog.ast.terms.Term
+
+abstract class Substitution(val from: Term, val to: Term) {
+    val mapped: Pair<Term, Term>? = if (from != to) from to to else null
+    override fun toString(): String = "$from -> $to"
+}
+typealias Substitutions = Map<Term, Term>
+typealias Answer = Result<Substitutions>
+typealias Answers = Sequence<Answer>