Result is ln(Number)
    which should be preferred for portability.
In coroutining mode, if Number is uninstantiated, the call to ln/2 is delayed until this variable is instantiated.
Success:
      ln(2.0, Result).          (gives Result = 0.693147)
      ln(1, Result).            (gives Result = 0.0)
Fail:
      ln(1, 1.0).
      ln(1, 0).
      ln(1, r).
Error:
      ln(A, 6.0).                   (Error 4).
      ln(-2, Result).               (Error 20).
      ln(0, Result).                (Error 20).
      ln(4 + 2, 1.79176).           (Error 24).