## UNITY--Chandy and Misra's UNITY formalism

The book Parallel Program Design: A Foundation by Chandy and Misra (Addison-Wesley, 1988) presents the UNITY formalism. UNITY consists of an abstract programming language of guarded assignments and a calculus for reasoning about such programs. Misra's 1994 paper "A Logic for Concurrent Programming" presents New UNITY, giving more elegant foundations for a more general class of languages. In recent work, Chandy and Sanders have proposed new methods for reasoning about systems composed of many components.

This directory formalizes these new ideas for UNITY. The Isabelle examples may seem strange to UNITY traditionalists. Hand UNITY proofs tend to be written in the forwards direction, as in informal mathematics, while Isabelle works best in a backwards (goal-directed) style. Programs are expressed as sets of commands, where each command is a relation on states. Quantification over commands using [] is easily expressed. At present, there are no examples of quantification using ||.

A UNITY assertion denotes the set of programs satisfying it, as in the propositions-as-types paradigm. The resulting style is readable if unconventional.

Safety proofs (invariants) are often proved automatically. Progress proofs involving ENSURES can sometimes be proved automatically. The level of automation appears to be about the same as in HOL-UNITY by Flemming Andersen et al.

The directory `Simple` presents a few examples, mostly taken from Misra's 1994 paper, involving single programs. The directory `Comp` presents examples of proofs involving program composition.