Parrying in 2nd Ed is rather lame (my opinion, of course) so I've
instituted this alternate:

THAC0 - 10 = Parrying AC.

Player's substitute this AC for their normal AC at the cost of one attack
in the round, only useful in melee and only against one attack per attack
sacrificed.

In this way a weapon master is more defensive, and low level people people
aren't unbalancing the game by being untouchable. I also allow the standar
Full Parry as described in the books at the cost of all actions in the
round (except to backpedal)