This book presents a unifying approach to semantical concepts and deductive methods used in recursive, equational and logic programming, data type specification and automated theorem-proving. The common background is Horn logic with equality. Although this logic does not cover the full first-order logic, it supplies us with a language that allows "natural" problem specifications, offers several semantical views (functional, relational, inductive, behavioural, etc.) and puts at our disposal a number of more or less special-purpose deductive methods, which can be used as rapid prototyping...