Implement 'positive?' and 'negative?' in Scheme

[Katie Burke]

positive? should return #t if real is greater than zero, #f otherwise.
negative? should return #t if real is less than zero, #f otherwise.
( http://www.scheme.com/tspl4/objects.html#./objects:s94 )

(positive? 128)

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

(negative? -65)

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