receive t ~filter ~postcond apply
postcond to the messages picked by
postcond returns true, otherwise keep trying until
determined, which will raise an exception.
If this returns successfully, the remaining data in the mailbox will be:
swallow is false.
swallow is true OR if no messages passed the filter.
clear t wipes out all previously received messages matching the
to_remove is not provided, wipes out all the messages.
Immediately after calling
zero t f succeeds for any