Result is +Number
    which should be preferred for portability.
Number and Result have to be of the same type.
In coroutining mode, if Number is uninstantiated, the call to +/2 is delayed until this variable is instantiated.
Success:
      +(1, 1).
      +(5, Result).        (gives Result = 5)
      +(-6.2, Result).     (gives Result = -6.2)
Fail:
      +(1, 0).
      +(1.0, 1).
      +(1, 1.0).
      +(1, r).
Error:
      +(A, 6).                   (Error 4).
      +(4 + 2, 6).               (Error 24).