Bockmayr, Alexander Programming Logics, MPI for Informatics, Max Planck Society;
https://rdcu.be/dr7aM (Publisher version)
Bockmayr, A. (1993). Conditional Narrowing Modulo a Set of Equations. Applicable Algebra in Engineering, Communication and Computing, 4(3), 147-168. doi:10.1007%2FBF01202035.