This is silly. If you really need the extra speed, then use a Scheme
   implementation with shared substrings, or another language (such as C).

Not an option for a portable library. We've been through this.

   >  ms> However, I have a compromise idea. Instead of the optional START
   >  ms> and END parameters, allow using a list '(s start end) instead of
   >  ms> the string parameter.
   >
   >I'm not sure I understand how this is a win over the optional
   >start/end parameters,

   The advantage is that you can store the string with the indices as one
   (conceptual) unit, thus making the program code easier to maintain and
   less error-prone.

You are simply re-inventing a form of shared-text strings, but one that is
particularly inefficient and that has poor error-checking properties. This I
do not want to do.

   Please don't destroy the nice high-level Scheme language with these
   exaggerated low-level optimazion efforts.

Try this: just ignore those optional parameters and you'll have the
interface you want.
    -Olin