34.9.7 Execution of Checking Indexicals

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

• all arguments of C except X should occur in R
• if R is ground in S, S(R) = \TV(X,C,S)

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

• If D(X,S) is contained in S(R), none of the possible values for X can make C false, and so C is detected as entailed.
• Otherwise, if D(X,S) is disjoint from S(R) and R is ground in S, all possible values for X will make C false, and so C is detected as disentailed.
• Otherwise, D(X,S) contains some values that could make C true and some that could make C false, and the indexical suspends.

A checking indexical is scheduled for execution as follows:

• it is evaluated initially as soon as it has become anti-monotone
• it is re-evaluated when one of the following conditions occurs:
  1. the domain of X has been pruned, or X has been assigned
  2. the domain of a variable Y that occurs as dom(Y) or card(Y) in R has been pruned
  3. the lower bound of a variable Y that occurs as min(Y) in R has been increased
  4. the upper bound of a variable Y that occurs as max(Y) in R has been decreased