SWI-Prolog -- #< / 2

Documentation
- Reference manual
  - The SWI-Prolog library
    - library(clpfd): CLP(FD): Constraint Logic Programming over Finite Domains
      - CLP(FD) predicate index
        
        Arithmetic constraints
        
        #=/2
        
        #\=/2
        
        #>=/2
        
        #=</2
        
        #>/2
        
        #</2
- Packages

Availability::- use_module(library(clpfd)).

?X #< ?Y

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 (section A.8.3).

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)].