"stored procedures"

So the topic of "real" "stored procedures" came up again.  Meaning a
function-like object that executes outside of a regular transaction,
with the ability to start and stop SQL transactions itself.

I would like to collect some specs on this feature.  So does anyone have
links to documentation of existing implementations, or their own spec
writeup?  A lot of people appear to have a very clear idea of this
concept in their own head, so let's start collecting those.