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