95% or more of the record-type definitions in practice are supposed to
be nongenerative.  The R6RS syntax, however, makes generative
record-type definitions the default and needs an extra clause
(nongenerative) to override this default.

We can change the R6RS version of `define-record-type' in SRFI 237 so
that non-generativity becomes the default.  For the few cases,
generative record types are needed, a clause (nongenerative #f) must
be added.

The SRFI 9 version of `define-record-type' would still create
generative record types.  We can't change this in SRFI 237 unless we
want to abandon R7RS Small compatibility (slightly).

What do you think?