backward-match in color:text<%>

• (send a-color:text backward-match position cutoff) -> (union natural-number? false?)
     position : natural-number?
     cutoff : natural-number?

  Skip all consecutive whitespaces and comments (using skip-whitespace) immediately preceeding the position. If the token at this position is a close, return the position of the matching open, or #f if there is none. If the token was an open, return #f. For any other token, return the start of that token.

  Must only be called while the tokenizer is started.