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  1. even(0).
  2. even(X) :- mod(X,2) is 0.
Success #stdin #stdout 0.02s 68352KB
stdin
mod(2,2).
stdout
GNU Prolog 1.3.1
By Daniel Diaz
Copyright (C) 1999-2009 Daniel Diaz
uncaught exception: error(existence_error(procedure,(mod)/2),top_level/0)