> An <sre> is not a data type. It is a language. This is like having
> 'valid-syntax?' to check an expression before you call 'eval' with it.

On this point, actually the SRE Syntax section of the SRFI draft states

"Note that an SRE is a first-class object consisting of nested lists of
strings, chars, char-sets, symbols and numbers. Where the syntax is
described as (foo bar), this can be constructed equivalently as '(foo
bar) or (list 'foo 'bar), etc."

Kevin Wortman