backward-containing-sexp in color:text<%>
( -> voidsend a-color:text backward-containing-sexp)
( -> (union natural-number? false?)
send a-color:text backward-containing-sexp position cutoff)
position : natural-number?
cutoff : natural-number?
Return the starting position of the interior of the (non-atomic)
s-expression containing position, or #f is there is none.
Must only be called while the tokenizer is started.