An ability to use {n, l, "something} ! Msg

Hi, Ulf. Since you thought that the idea is worth a try, I decided to make a pull request instead :)

The implementation I've mentioned at the issues was incomplete, so don't mind it. The code of parse transform is completely stolen from erl_id_trans.erl, I've just added 2 lines:
https://github.com/doubleyou/gproc/blob/master/src/gproc_pt.erl#L450