Unison fails to remove lock files when it is interrupted.  This causes
subsequent attempts to fail.
	Fatal error: Warning: the archives are locked.
	If no other instance of unison is running, the locks should be
	removed.

Other programs put the process ID in the lock file.  Unison puts a lot
of stuff in the lock files but it does not appear that the process ID is
there.  Adding it would allow Unison to detect whether or not the
locking process is still alive (if on the same machine).  If it's not,
the lock should be removed and disregarded.

Thank you.