If you're pretty comfortable with Scheme, or are
feeling particularly adventurous, check out the (match ...) syntax
in the
PLT Scheme Docs. It could make your parser prettier and easier to write.
Note that this a power user feature and is not required. If
you find yourself getting frustrated using the cond/case syntax to
parse the input, take a look. If you're still shaky on Scheme in general, skip
it for now.