I have a silly question, looking at NetBSD source code, each file has a 
tag on the first line, like the one below

$NetBSD: kern_lock.c,v 1.83 2004/08/04 10:37:08 yamt Exp $

Could somebody please explain the format of those tags, i.e. why is 
there '$' at the start and end? What is 'Exp' for? What is 'yamt' for?

Thanks