Result is atan(Number)
    which should be preferred for portability.
In coroutining mode, if Number is uninstantiated, the call to atan/2 is delayed until this variable is instantiated.
Success:
      atan(0.0, 0.0).
      atan(1.0, Result).       (gives Result = 0.785398)
      atan(-8, Result).        (gives Result = -1.44644)
Fail:
      atan(1, 0.0).
      atan(1.55741, 1).
      atan(5, r).
Error:
      atan(A, 6.0).                   (Error 4).
      atan(1 + 0.55741, 1.0).         (Error 24).