Alternatives to the WAM

Question

I remember once reading that there were at least two other alternatives invented roughly at the same time as the WAM. Any pointers?

Answer 1

Not sure if this is what you mean, but the first two Prolog implementations were an interpreter written in Fortran by Colmerauer et al. and a DEC PDP-10 native compiler by Warren et al.

Warren mentions these in his foreword to Ait-Kaci's Tutorial Reconstruction of the WAM. If this is not what you mean, you may find it in that document or its references.