[PD] externals that send/receive messages?

[Forrest Curo]

There is a little about this in the 'writing externals' instructions
( http://iem.at/pd/externals-HOWTO/pd-externals-HOWTOse6.html#x8-37000A )

but I'm not finding anything that makes sense to me as to how to code that
into an external object. Can anyone point me to examples & fuller
explanations?
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