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 or equal to Y. When reasoning over integers, replace (=<)/2 by #=</2 to obtain more general relations. See declarative integer arithmetic (section A.9.3).