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syntax: (define-constraint <constraint-name> OF
            [:DOCUMENTATION <constr.doc.>]
            [:EXPLANATION <expl-expr.>]
            (<bab-var> <bab-lisp-expression>)]
            WITH <simple-predication>+)

Warning: The explanation facility is not sup-
         ported, hence :documentation and
         :explanation are ignored.

With a constraint you define expressions for
the parameters of the constraint-pattern. In the
constraint-pattern is defined, how to link
certain values. The constraint defines
the concrete values for the link. You can link
attribut-values of one or more instances.

FOR-EACH expects a list with a variable which is
followed by a lisp-expression. The lisp-expression
has to evaluate to a list with valid conditions.
For example: With the FOR-EACH declaration it is
possible to make a constraint for every instance
of a certain frame.

WITH expects a simple predication. The predications
in the WITH-clauses must not be based upon each other.
There must exist exactly one WITH-clause for every
parameter of the pattern.

example: >(define-constraint-pattern
             (_real-costs _country-wishes)
             :documentation "calculates the
             costs for a visit of the countries
             (KNOWN (_country-wishes)
             THEN (is _real-costs (calculate-client-
             costs _country-wishes))))
         >(define-constraint calculate-costs-for-
             OF calculate-real-costs
             FOR-EACH (a-client (instances-of
             [complete-costs a-client _real-costs]
             [destinations a-client _country-wishes])

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