Major mode for editing DSSSL code.
Editing commands are similar to those of `lisp-mode'.
Delete converts tabs to spaces as it moves back.
Blank lines separate paragraphs. Semicolons start comments.
Uses keymap `scheme-mode-map', which is not currently defined.
Entering this mode runs the hooks `scheme-mode-hook' and then
`dsssl-mode-hook' and inserts the value of `dsssl-sgml-declaration' if
that variable's value is a string. (fn)