Ask-Tell
The two communication primitives of Concurrent Constraint Programming. Tell c → P (Saraswat’s atomic tell) adds constraint c to the constraint store if it is consistent with what is already there, then continues as P; the simpler eventual tell variant adds unconditionally without consistency-checking. Ask c ⊃ P blocks until the store entails c, then continues as P — synchronous on entailment. All other concurrency operators (parallel composition, choice, hiding, recursion) are built from ask, tell, and the algebraic structure of the constraint system.