Title:
Leads-to Properties and the But-not-yet Operator

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Author(s)
Calvert, Kenneth L.
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Abstract
We define a predicate transformer, in terms of which finite disjunctions of leads-to properties can be rewritten as single leads-to properties. Although disjunctions of leads-to properties do not typically arise naturally in progress specifications, an example shows how they may be introduced through the use of nested implications of leads-to properties; such implications allow subtle dependencies between a program's progress and that of its environment to be conveniently specified. After introducing the predicate transformer, which is called the "but-not-yet" operator, we show how to define a single leads-to property equivalent to a given disjunction of two leads-to properties. An alternative definition of the but-not-yet operator is shown to be equivalent to the first, and some properties of the operator are proved. Finally, the predicate transformer is generalized to any finite number of arguments.
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Date Issued
1993
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149205 bytes
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Text
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Technical Report
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