[Retros] Retractor 2.1 released

[Theodore Hwa]

Hi all,

Retractor 2.1 has been released, at the same location:
http://xenon.stanford.edu/~hwatheod/retractor2/retractor.html

Improvements made:

1. Retractor can now detect an illegally placed king behind enemy pawns. 
For example: L. Ceriani, 3bkN1K/pppprp1p/4p1p1/8/8/8/8/8, Last move?

Because Retractor can now detect that retracting g7-g6 leaves the white 
king illegally placed, all moves except the correct answer can be 
eliminated.

2. Retractor can detect promoted rooks outside of a cage. For example: H. 
Cross, Q1n1Kb2/B1kppppp/1prp4/2p5/8/8/8/8, Last move?

Retractor can now find the correct answer by detecting that b7xRa8=Q is 
impossible, as one of the black rooks would have to be promoted due to the 
cage in the northeast corner.

Among the "Last Move" test problems, Retractor 2.1 solves 12 additional 
problems compared to Retractor 2.0.

Enjoy!

Ted