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, 7).
      +(5, -2.0, 3.0).
Fail:
      +(1, 2, 7).
      +(1, 2, 3.0).
      +(5, 2, r).
Error:
      +(A, 2, 6).             (Error 4).
      +(7 - 4, 2, 3).         (Error 24).