[Retros] Add units: 13th update

[Mu-Tsun Tsai]

This is a very trivial "1KA" problem using 23 pieces.
Obviously nobody bother to mention this one so far.

FEM: 2bqkb2/1pppppp1/8/2p2p2/2P2P2/8/1PPPPPP1/2BQ1B2

However, I do wonder if it is possible to use fewer pieces to force a 1KA 
problem. I did try, but haven't succeeded yet.
Anyone have an idea?

Mu-Tsun Tsai