Result is abs(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 abs/2 is delayed until this variable is instantiated.
Success:
      abs(1, 1).
      abs(-5, Result).             (gives Result = 5)
      abs(-6.2, Result).           (gives Result = 6.2)
Fail:
      abs(1, 0).
      abs(1, 1.0).
      abs(-1.0, 1).
      abs(1, r).
Error:
      abs(A, 6).                   (Error 4).
      abs(4 + 2, 6).               (Error 24).