The arithmetic expression X is less than Y. When reasoning over
integers, replace (<)/2 by #</2 to obtain more general relations. See
declarative integer arithmetic.
In addition to its regular use in tasks that require it, this
constraint can also be useful to eliminate uninteresting symmetries
from a problem. For example, all possible matches between pairs
built from four players in total:
?- Vs = [A,B,C,D], Vs ins 1..4,
all_different(Vs),
A #< B, C #< D, A #< C,
findall(pair(A,B)-pair(C,D), label(Vs), Ms).
Ms = [ pair(1, 2)-pair(3, 4),
pair(1, 3)-pair(2, 4),
pair(1, 4)-pair(2, 3)].
