10.34.9.6 Execution of Propagating Indexicals

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:

• 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 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:

• If D(X,S) is disjoint from S(R), a contradiction is detected.
• If D(X,S) is contained in S(R), D(X,S) does not contain any values known to be incompatible with C, and the indexical suspends, unless R is ground in S, in which case C is detected as entailed.
• Otherwise, D(X,S) contains some values that are known to be incompatible with C. Hence, X::S(R) is added to the store (X is pruned), and the indexical suspends, unless R is ground in S, in which case C is detected as entailed.

A propagating indexical is scheduled for execution as follows:

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