6.2  Basic Constraints

The following constraints implement the basic relationships between two domain values. The constraints require their arguments to come from identical domains, otherwise an error is raised.

?X &= ?Y

X is the same as Y

?X &\= ?Y

X is different from Y

?X &< ?Y

X is strictly before Y in the domain order

?X &> ?Y

X is strictly after Y in the domain order

?X &=< ?Y

X is the same as Y, or before Y in the domain order

?X &>= ?Y

X is the same as Y, or after Y in the domain order

shift(?X,?C,?Y)

Y is C places above X in the domain order. X and Y have symbolic domains, C has an integer domain.

rotate(?X,?C,?Y)

like shift/3 but wraps at domain boundary.

element(?Index,++List,?Value)

Value occurs in List at position Index. Value has a symbolic domain, Index has an integer domain. List is a number of symbolic domain values.

For example

?- [X, Y] &:: weekday, X &< Y.
X = X{[mo, tu, we, th, fr, sa]}
Y = Y{[tu, we, th, fr, sa, su]}
Yes (0.00s cpu)

?- X &:: weekday, X &=< we.
X = X{[mo, tu, we]}
Yes (0.00s cpu)