Result is sqrt(Number)
    which should be preferred for portability.
In coroutining mode, if Number is uninstantiated, the call to sqrt/2 is delayed until this variable is instantiated.
Success:
      sqrt(1.0, 1.0).
      sqrt(1.0, Result).      (gives Result = 1.0)
      sqrt(49, Result).       (gives Result = 7.0)
Fail:
      sqrt(1, 0.0).
      sqrt(1, 1).
      sqrt(1, r).
Error:
      sqrt(A, 6.0).                   (Error 4).
      sqrt(-2, Result).               (Error 20).
      sqrt(4 + 2, 2.44949).           (Error 24).