Next: Execution of Checking Indexicals, Previous: FD Predicates, Up: Defining Primitive Constraints [Contents][Index]

Consider the definition of a constraint `C` containing a propagating
indexical

. Let `X` in `R``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, 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

thus involves the following:
`X` in `R`

- If
`D(X,S)`is disjoint from`S(R)`, then a contradiction is detected. - If
`D(X,S)`is contained in`S(R)`, then`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,`S`is extended to`(X in S(R))(S)`(`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:
- the domain of a variable
`Y`that occurs as`dom(`

or`Y`)`card(`

in`Y`)`R`has been updated - the lower bound of a variable
`Y`that occurs as`min(`

in`Y`)`R`has been updated - the upper bound of a variable
`Y`that occurs as`max(`

in`Y`)`R`has been updated

- the domain of a variable

Send feedback on this subject.