[Retros] Add units: 13th update

[Mu-Tsun Tsai]

Hello everyone,

My name is Mu-Tsun Tsai and I'm from Taiwan. I just joined this mailing list 
today, and I'm pretty new to retro compositions. As I'm reading this article 
about the record of "add units" type problem, I'm kind of impressed that 
there're no record for adding 1 unit Type A problem except for N1KAi. So I 
tried to compose this one. This one is probably not very economic, but at 
least we might have something to begin with.

FEN: nNb5/nQrppppp/qRp5/kp6/p6P/P1P5/RPrPPPP1/K7

This should be a "1NAi" problem, if I'm correct about the design 
(hopefully... I'm new here, you know...). The position is illegal, neither 
side is in check, and without indicating which side has the move or how many 
units one should add, the only solution is to add a white knight at b1 and 
retract -1...Rc1 -2. c2 Rh1 -3. Nc3 etc.
Please let me know if this one works! Thanks :)

Mu-Tsun Tsai