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>