About return values:

* SRFI authors don't annotate return values consistently.
   Often they are left out of the signature, and only discussed
   in prose in the description.

* So if the tool extracts a return value from the signature, we can
   assume that it's correct. But if it doesn't extract any return value,
   we can't assume anything about what the procedure returns.

* Some procedures return multiple values as a list. At least SRFI 1 has
   many of these and they are marked up with square brackets like [a b].
   The tool currently treats this as two values instead of one list.
   This can be changed; the right approach is up for debate. The current
   approach gives output like this:

     ;; span  pred clist -> [list clist]

     (procedure span
       (arg pred)
       (arg clist)
       (return list)
       (return clist))