[Pymilter] minimal working example of pymilter script

[Fourhundred Thecat]

Hello,

I would like to process mail that arrives at my Postfix server via smtpd
(SMTP-only filter).

I am new to milters, and I am looking for simplest possible working
example of a python3 pymilter script. Something that does nothing
useful, perhaps just adds "hello world" at the beginning of the message
body.

All the examples that I could find online are overcomplicated and
incomprehensible to my beginner self.

Could some kind person link me to a minimal working example of pymilter
script ?

thanks,