Result is sgn(Number)
    which should be preferred for portability.
sgn/2 gives the integer -1 if Number is negative, 0 if it is zero and 1 if it is greater than zero. It is always true that
    X =:= sgn(X) * abs(X)
   In coroutining mode, if Number is uninstantiated, the call to sgn/2 is
   delayed until this variable is instantiated.
Success:
      sgn(42, 1).
      sgn(-5, Result).             (gives Result = -1)
      sgn(-6.2, Result).           (gives Result = -1)
      sgn(0.0, 0).
Fail:
      sgn(1, 0).
      sgn(1, 1.0).
      sgn(1, r).
Error:
      sgn(A, 6).                   (Error 4).
      sgn(4 + 2, 6).               (Error 24).