P listens for these ACKs and broadcasts a report of all the guys it has heard ACKS from. After a guy sends out an ACK to a target, it listens for the report from the target, as a confirmation that 2-way communication has been established up between it and the target. There is the beginning of some code to do this, but this is not yet debugged, so you should use it as an example of a possible language, not a working algorithm. You can snarf the files in the directory and play with them.
Some minimal instructions are at the top of hpm-setup.