Equality, types and generic modules for logic programming. A language using Horn clauses. J.A. Goguen, J. Meseguer.

OBJ2 plus logic programming based on Horn logic with equality.

"EQLog: Equality, Types and Generic Modules for Logic Programming", J. Goguen et al in Functional and Logic Programming, D. DeGroot et al eds, pp.295-363, P-H 1986.