Prolog was introduced into general public in 1972, 4 years after Hindley's algorithm that used unification and substitution. In any case, what was suggested is

Prolog was introduced into general public in 1972, 4 years after Hindley's algorithm that used unification and substitution. In any case, what was suggested is, frankly, a development of the (tailored up) type system. My experiece suggests that this development will not be without caveats, especially related to performance.