This page is part of the web mail archives of SRFI 115 from before July 7th, 2015. The new archives for SRFI 115 contain all messages, not just those from before July 7th, 2015.
It looks like if literal character sets were changed from (<string>) to something like (literal-charset <string>), then sequences would not need to start with : or seq. The grammar for sequences could change from (: <sre> ...) to (<sre> ...). I think that it would look cleaner, but then I am something of a heretic.
For consistency with R7RS #!fold-case and #!no-fold-case, I suggest changing these to be w/no-fold-case and w/fold-case.