MOLOG: a Modal PROLOG

Modal logics (or more generally non-classical logics) enable us to express concepts of belief, knowledge or assumption thanks to modal operators qualifying classical formulas. Hence MOLOG handles PROLOG clauses qualified by modal operators. Formally we extend the definition of Horn clauses to modal logics. A clause F is a Modal Horn Clause if F is a formula such that if we erase all the modal operators in F the remaining clause is a Horn clause.