CycL

CycL in computer science and artificial intelligence is an ontology language used by Doug Lenat's Cyc artificial intelligence project. Ramanathan V. Guha was instrumental in the design of early versions of the language. There is a close variant of CycL known as MELD.

The original version of CycL was a frame language, but the modern version is not. Rather, it is a declarative language based on classical first-order logic, with extensions for modal operators and higher order quantification.

CycL is used to represent the knowledge stored in the Cyc Knowledge Base, available from Cycorp. The source code written in CycL released with the OpenCyc system is licensed as open source, to increase its usefulness in supporting the semantic web.

CycL has some basic ideas:

The concept names in Cyc are known as constants. Constants start with "#$" and are case-sensitive. There are constants for:

The most important predicates are #$isa and #$genls. The first one (#$isa) describes that one item is an instance of some collection (i.e.: specialization), the second one (#$genls) that one collection is a subcollection of another one (i.e.: generalization). Facts about concepts are asserted using certain CycL sentences. Predicates are written before their arguments, in parentheses:

For example:

"Bill Clinton belongs to the collection of U.S. presidents" and

"All trees are plants".

"Paris is the capital of France."

Sentences can also contain variables, strings starting with "?". One important rule asserted about the #$isa predicate reads

with the interpretation "if OBJ is an instance of the collection SUBSET and SUBSET is a subcollection of SUPERSET, then OBJ is an instance of the collection SUPERSET".

...
Wikipedia