Quite so. Unfortunately, we are in a chicken and egg position at
present. The types of Scheme have been stable since R2RS, but things
will change in R7RS-large. For example, if SRFI 113 sets are adopted,
then `set?` will be a type predicate of Scheme. It would be absurd to
create a situation in which objects returned by a SRFI 137 constructor
were guaranteed to answer `#f` to `symbol?`, but might answer either
`#t` or `#f` to `set?`. We won't be able to make a new enumeration of
Scheme's types until R7RS-large is complete, or at least until it is
much further along than it is today. So I'm basically stuck with using
vague language which will all come together in the Ultra-Violet Edition.

Maybe SRFI 137 itself can serve as an anchor or reference point for future SRFIs.

E.g., an imaginary SRFI 138 about, say, a data structure named heap, could specify a procedure heap? "that behaves as if it was returned as the third argument to SRFI 137's make-type".

Marc