Result is Number1 / Number2
    which should be preferred for portability.
The result type of the division depends on the value of the global flag prefer_rationals. When it is off, the result is a float, when it is on, the result is a rational. In coroutining mode, if Number1 or Number2 are uninstantiated, the call to //3 is delayed until these variables are instantiated.
Success:
    /(10, 2, 5.0).
    /(10, -2.0, -5.0).
    /(9, 12, 3_4).      (with set_flag(prefer_rationals, on))
Fail:
    /(1, 2, 1.0).
    /(5, 2, r).
    /(6, 2, 3).
Error:
    /(A, 2, 6.0).            (Error 4).
    /(2, 0, Result).         (Error 20).
    /(4 + 2, 2, 12).         (Error 24).