Result is Number1 - Number2
    which should be preferred for portability.
The result is of type breal if any of the arguments is a breal, else float if any of the arguments is a float, else rational if any of the arguments is a rational. Only when both arguments are integers is the result an integer.
In coroutining mode, if Number1 or Number2 are uninstantiated, the call to -/3 is delayed until these variables are instantiated.
Success:
      -(5, 2, 3).            (gives Result = 3)
      -(5, -2.0, Result).    (gives Result = 7.0)
Fail:
      -(1, 2, 3).
      -(1, 2, 3.0).
      -(5, 2, r).
Error:
      -(A, 2, 6).             (Error 4).
      -(4 + 1, 2, 3).         (Error 24).