logic rules

Logic rules are typically written as implications of the form if X, Y, Z , .. are all true then infer A, B, C ..., where A, B, C, ...X, Y, Z are all logical statements potentially including variables. The individual statements may vary in complexity, for example simple propositional logic, simple predicate caculus or higher-order logics allowing precicates to about predicates. In the logic programing language Prolog the right hand side (written on the left in Prolog!) is limited to a single first-order propostion with no ANDs or ORs, that is a Horn clause.