Constraint Logic Programming (CLP) augments Prolog by adding
constraints to the clauses. The CLP implementation solves goals in the
same manner as Prolog, but also merges the constraints associated with
each rule. If the merge succeeds, the successful goal and the
corresponding constraints are returned. If, however, the constraints
are mutually exclusive, the solution fails.

There is an extensive wide-ranging archive of material relating to
constraint logic programming at
   http://web.cs.city.ac.uk/archive/constraints/constraints.html