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, -1.0).
      -(1.0, -1).
      -(1, r).
Error:
      -(A, 6).                   (Error 4).
      -(4 + 2, -6).              (Error 24).