[racket] section 11.5 from htdp

I am having some difficulties with section 11.5 from htdp. I'm of course not
the first one encountering such problems:

http://lists.racket-lang.org/users/archive/2008-November/028325.html

but the above thread doesn't help in any way - Mr. Felleisen confirms that
one shouldn't assume that x is a natural number. But in his next post he
says that the function multiply from previous exercises should be used. Am I
missing or misunderstanding something? Is it possible to multiply x by x n
times with my multiply function when it needs both arguments to be natural
numbers? My first idea was to use a different function, say
multiply-by-inexact, which uses the + operator like multiply-by-pi, but it
still needs one natural argument to stop recursion. In which direction
should I go to solve this? My exponent function works for natural numbers,
but I don't want to miss anything important while going through this
textbook.

Krzysztof
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