I am having some difficulties with section 11.5 from htdp. I'm of course not the first one encountering such problems: <a href="http://lists.racket-lang.org/users/archive/2008-November/028325.html">http://lists.racket-lang.org/users/archive/2008-November/028325.html</a> 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