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There are four HOL systems (sharing essentially the same logic) that are still maintained and developed.
- The first, HOL4 stems from the HOL88 system, which was the culmination of the original HOL implementation effort, led by Mike Gordon. HOL88 included its own ML implementation, which was in turn implemented on top of Common Lisp. The implementations following HOL88 (HOL90, hol98 and HOL4) all used Standard ML as the implementation language. The hol98 system is tied to the Moscow ML implementation of Standard ML; HOL4 can be built with either Moscow ML or Poly/ML. Of these four systems, only HOL4 is being maintained and developed. All come with large libraries of theorem proving code. These implement extra automation on top of the very simple core code. HOL4 is BSD licensed.
- The second current implementation is HOL Light. This started as an experimental "minimalist" version of HOL. Although it has subsequently grown into another mainstream HOL variant, its logical foundations remain unusually simple. HOL Light used to be implemented in Caml Light, but now uses OCaml. HOL Light is available under the new BSD license.
- The third current implementation is ProofPower a collection of tools designed to provide special support for working with the Z notation for formal specification. 5 of the 6 tools are GNU GPL v2 licensed. The sixth (PPDaz) has a proprietary license.
- The fourth is HOL Zero, a minimalist implementation focused on trustworthiness. HOL Zero is GNU GPL 3+ licensed
HOL is a predecessor of Isabelle.
Read more about this topic: HOL (proof Assistant)
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