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<HTML><HEAD><TITLE>HOL/UNITY/README</TITLE></HEAD><BODY>
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<H2>UNITY--Chandy and Misra's UNITY formalism</H2>
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<P>The book <EM>Parallel Program Design: A Foundation</EM> by Chandy and Misra
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(Addison-Wesley, 1988) presents the UNITY formalism. UNITY consists of an
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abstract programming language of guarded assignments and a calculus for
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reasoning about such programs. Misra's 1994 paper "A Logic for Concurrent
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Programming" presents New UNITY, giving more elegant foundations for a more
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general class of languages. In recent work, Chandy and Sanders have proposed
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new methods for reasoning about systems composed of many components.
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<P>This directory formalizes these new ideas for UNITY. The Isabelle examples
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may seem strange to UNITY traditionalists. Hand UNITY proofs tend to be
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written in the forwards direction, as in informal mathematics, while Isabelle
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works best in a backwards (goal-directed) style. Programs are expressed as
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sets of commands, where each command is a relation on states. Quantification
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over commands using [] is easily expressed. At present, there are no examples
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of quantification using ||.
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<P>A UNITY assertion denotes the set of programs satisfying it, as
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in the propositions-as-types paradigm. The resulting style is readable if
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unconventional.
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<P>
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The directory presents a few small examples, mostly taken from Misra's 1994
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paper:
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<UL>
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<LI>common meeting time
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<LI>the token ring
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<LI>the communication network
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<LI>the lift controller (a standard benchmark)
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<LI>a mutual exclusion algorithm
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<LI><EM>n</EM>-process deadlock
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<LI>unordered channel
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<LI>reachability in directed graphs (section 6.4 of the book)
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</UL>
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<P> Safety proofs (invariants) are often proved automatically. Progress
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proofs involving ENSURES can sometimes be proved automatically. The
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level of automation appears to be about the same as in HOL-UNITY by Flemming
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Andersen et al.
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<HR>
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<P>Last modified on $Date$
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<ADDRESS>
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<A NAME="lcp@cl.cam.ac.uk" HREF="mailto:lcp@cl.cam.ac.uk">lcp@cl.cam.ac.uk</A>
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</ADDRESS>
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</BODY></HTML>
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