reader-close and writer-close seem missing from the current draft.

It may also be good to note that the 'close' procedure passed to
make-simple-reader/make-simple-writer is called only synchronously,
when reader-close/write-close procedure is called.

(Another option is to allow an implementation to call 'close' when
the reader/writer is about to be GC-ed, but that'd make it difficult
to write portable code using srfi-68).

--shiro