Result is rational(Number)
    which should be preferred.
When Number is an integer, Result is a rational with denominator 1.
When Number is already a rational, Result is identical to Number.
When Number is a float, Result is a rational whose value is exactly equal to the value of the floating-point number. Since floats are usually approximations of the intended value, the results may look unintuitive and have unnecessarily large numerators and denominators. Use rationalize/2 to produce the most compact rational that still converts back into the original float. rational/2 is usually faster than rationalize/2.
Bounded reals cannot be converted to rationals.
In coroutining mode, if Number is uninstantiated, the call to rational/2 is delayed until this variable is instantiated.
Success:
      rational(25, 25_1).
      rational(1.5, 3_2).
      rational(3_4,3_4).
      rational(9_12,3_4).
      rational(-6, Result).      (gives Result = -6_1)
      rational(0.1, Result).     (gives Result = 3602879701896397_36028797018963968)
Fail:
      rational(1, 2_1).
      rational(3, 3).
      rational(1, r).
Error:
      rational(A, 1_3).                   (Error 4).
      rational(4 + 2, 6_1).               (Error 24).
      rational(0.9__1.1, X).              (Error 141).