1. Introduction

Abstract

Certainly, the ability to draw inferences is a central operation in any Artificial Intelligence (AI) system. Consequently, automated deduction is one of the most traditional disciplines in AI. A major goal of research here is to develop general algorithms or semi-decision procedures for the solution of problems formulated in first-order predicate logic. One core technique, the resolution principle [Robinson, 1965b] nowadays seems to be even standard knowledge of any computer scientist. Resolution made popular the use of clause logic and of unification in theorem proving calculi. In particular, the use of unification is a major point, as calculi were predominated at that time by reducing first order logic to propositional logic by more or less blindly guessing ground instantiations of formulae.