/usr/lib/swipl/library/ext/clib/unix.pl
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 fork_exec(+Command) is det
Fork (as fork/1) and exec (using exec/1) the child immediately. This behaves as the code below, but bypasses the check for the existence of other threads because this is a safe scenario. 
fork_exec(Command) :-
      (   fork(child)
      ->  exec(Command)
      ;   true
      ).