A system for specifying, designing, and verifying the design of digital systems; it was devised at Cambridge University, UK. The HOL system includes a theorem prover, an editor, and consistency checkers. The HOL specification language has basic types that handle n-bit digital numbers, the natural numbers, and the Boolean values true/false. More complex types can be built by binding together the basic types. A library of functions and list operators is used to perform transformations on variables.