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Consider the definition of a constraint C containing a propagating indexical X in R. Let TV(X,C,S) denote the set of values for X that can make C true in some ground extension of the store S. Then the indexical should obey the following coding rules:

If the coding rules are observed, then S(R) can be proven to contain TV(X,C,S) for all stores in which R is monotone. Hence it is natural for the implementation to wait until R becomes monotone before admitting the propagating indexical for execution. The execution of X in R thus involves the following:

A propagating indexical is scheduled for execution as follows:

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