You need to import the constraint logic module. :- use_module(library(clpfd)). Goes at the top of the file (leave off the :- if you're in the repl). The ar

You need to import the constraint logic module. :- use_module(library(clpfd)). Goes at the top of the file (leave off the :- if you're in the repl). The arithmetic built in to prolog is traditionally of the form A is 1+2, B is A+3, ... etc.