Implement 'numerator' and 'denominator' in scheme

[Katie Burke]

numerator should return the numerator of a rational number.
denominator should return the denominator of a rational number.

This should return 2.

(numerator 2/3)

Traceback (most recent call last):
File "stdin", line 1, col 2
RunTimeError: unbound variable 'numerator'

This should return 1.

(denominator 9)

Traceback (most recent call last):
File "stdin", line 1, col 2
RunTimeError: unbound variable 'denominator'