I've thought of adding:

(ascii-opening-bracket char) => char or #f
(ascii-closing-bracket char) => char or #f
(ascii-opposing-bracket char) => opposing-char or #f

If char is one of ( [ { < then ascii-opening-bracket returns that same
char, otherwise it returns #f. For those same chars
ascii-opposing-bracket returns ) ] } > or #f respectively.

If char is one of ) ] } > then ascii-closing-bracket returns that same
char, otherwise it returns #f. For those same chars
ascii-opposing-bracket returns ( [ { < or #f respectively.

I've wished for procedures like these when writing parsers a couple of
times. Would this be too fancy? You can write code like this:

(if (ascii-opening-bracket char)
     (read-delimited-list (ascii-opposing-bracket char)))