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

?X #= ?Y

The arithmetic expression X equals Y. This is the most important arithmetic constraint (section A.8.2), subsuming and replacing both (is)/2 and (=:=)/2 over integers. See declarative integer arithmetic (section A.8.3).