[plt-scheme] String functions in Dr. Scheme

I have some experience with Common Lisp and emacs' elisp, but am new
to Scheme.  I'm trying to fool around with some functions I wrote in
elisp, that need to be able to map the characters in a string to their
numerical places in the alphabet.  It's no great surprise that string
handling is utterly different in Scheme than it is in elisp, but I'm
not finding the docs on just what string-handling functions are
defined in Dr. Scheme.  And of course, at what language level I have
to be in order to get at them.

Maybe I just need a pointer to the right doc page?  Which is a little
surprising, because mostly the PLT web interface looks remarkably
complete and well thought-out.

-jmc