Result is cos(Number)
    which should be preferred for portability.
In coroutining mode, if Number is uninstantiated, the call to cos/2 is delayed until this variable is instantiated.
Success:
      cos(0, 1.0).
      cos(1.2, Result).        (gives Result = 0.362358)
      cos(-33, Result).        (gives Result = -0.0132767)
Fail:
      cos(1, 0.0).
      cos(0, 1).
      cos(5, r).
Error:
      cos(A, 6.0).                   (Error 4).
      cos(4 + 2, 0.96017).           (Error 24).