# [1-18] Explanation of constraint entailment

```> Could anybody give me the definition of constraint entailment?

If we already have `X = 8' in the constraint store, we can ask three
questions about some other constraint C:

a) Is C entailed by the store?
If C is, say, `X > 5' then the answer is `yes' -- X > 5 does
not give you any extra information if you already know that
X = 8.
b) Is the negation of C entailed by the store? (Is C `disentailed')
If C is `X > 5' the answer is `no', but if C was, say, X < 2,
it would be false in the context provided by the store
c) Is C neither entailed nor disentailed?
The constraint `Y = 4' is consistent with the store, and so is
its negation, but it is not entailed by it.

In the language of Concurrent Constraints, we can `ask' if a constraint
is entailed, and get `yes', `no' or `suspend' (i.e. the ask goes to
sleep until enough extra information is added to the store to give a
definite answer). If we want to add information to the store, we can do
a `tell' which only works if the constraint to be told is not
inconsistent with the store.
```
Go Back Up