On the subject of experimental implementations of some of
the proposals, s7 has Alaric Snell-Pyms's make-encapsulated-type
under the name make-type:

https://ccrma.stanford.edu/software/snd/snd/s7.html#maketype

A tarball is at ftp://ccrma-ftp.stanford.edu/pub/Lisp/s7.tar.gz.

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