F-logic in the following manner: These mean "X is a man if X is a person but not a woman" and "if X is the son of Y then X is a person and Y is the father of X". (Note: the latter inference is not factual, because in addition to fathers, sons also have mothers. The gender of X is determined by the premise, but the gender of Y is not.) The Flora-2 system introduced a number of changes to the syntax of F-logic, making it more suitable for a knowledge representation and reasoning system as opposed to just a theoretical logic. In particular, variables became prefixed with a ?-mark, the distinction between functional and multi-valued properties was dropped and replaced by cardinality constraints, plus other important changes. For instance, the above pair of sentences look like this in Flora-2: Here is default negation ("naf" stands for Negation as failure, based on the well-founded semantics); Flora-2 also supports so-called explicit negation, which is closer to negation in classical logic. F-logic based languages Flora-2 is an extension of F-logic with HiLog, Transaction logic, and defeasible reasoning. Ergo is a commercial system based on F-logic, which extends Flora-2. PathLP is a full logic programming language based on F-logic. FLORID is a C++ — based implementation Web Services Modeling Language (WSML) Semantic Web Services Language (SWSL) ObjectLogic language is based on F-logic; OntoStudio is an ObjectLogic implementation by semafora systems GmbH (formerly Ontoprise GmbH). OO-logic is a successor of F-logic; SemReasoner and Ontology Management System (OSM) are implementations of OO-logic by adesso SE. References Knowledge representation Semantic Web Logic programming languages Declarative programming languages