add_sentusers_msg() is also used to handle acknowledgement operations. As such it takes another argument "ack", which is not documented in the code yet.
I believe "Purpose" is not completely correct anymore, since the acknowledgement author is excluded.
Other comments in escalator.c have a deviating spelling for acknowledgment/acknowledgement from the frontend. I think this should be consistent. The same kind of problem actually exists in a number of files.