Responses to ace asking bids of 4C, 4N, minorwood, or kickback are artificial
responses and are therefore prone to lead-directing doubles.  Sometimes, slam
is not safe even though you are missing only one keycard because the lead
directing double allows the defense to setup a trick which can later be taken
once the missing keycard is lost.  The following convention allows you to
determine your side's holding in the doubled ace asking responses.  The key
point to note is that if you are missing two keycards you make your normal
rebid of 5 of the trump suit.

Key:
T = trump suit
Y = ace asking suit or NT
Z = ace asking response

4Y (ace asking) - (P) - 4/5Z - (X):
   P:   Missing only one keycard.
        Denies first round control in suit Z.  Still interested in slam but
        leaving the decision up to pd.
   XX:  Missing only one keycard.
        Has first round control in suit Z but still worried about the suit.  
        Typically Ax or Axx.  Typically not a void in suit Z.
   5T:  signoff, missing 2 keycards/aces
   6T:  not worried about suit Z.  Could be singleton A or Kx or KQx.