This proposal replaces output-age priority with something we call “youngest-input” priority. Basically, the exit priority of an output is now the age of its youngest input. We claim that this construction is safe (honest clients cannot have money stolen) as long as clients don’t spend any outputs included after any invalid or witheld transaction.
We show that if there exists an “out of nowhere” input, then any transactions stemming from that input would have an even higher queue number than the “out of nowhere” itself. Therefore, valid outputs will be processed first.