Result is Number1 >> Number2
    which should be preferred for portability.
The shift behaves as an arithmetic (signed) shift operating on an unlimited length two's complement representation. Shifting by a negative amount is the same as shifting by the same positive amount in the other direction.
In coroutining mode, if Number1 or Number2 are uninstantiated, the call to >>/3 is delayed until these variables are instantiated.
Success:
      >>(8, 3, 1).
      >>(17, 3, X).          (gives X = 2)
Fail:
      >>(1, 2, 3).
      >>(16, 2.0, 4.0).
      >>(5, 2, r).
Error:
      >>(A, 2, 6).              (Error 4).
      >>(4 + 12, 2, 4).         (Error 24).