Show HN: Chess in Elm

Very nice and it plays ok, but I found a couple of bugs.

1. No castling.

2. Opponent made illegal move into check:

g4 d5 Bg2 Bxg4 f3 Bf5 e4 dxe4 fxe4 Bg6 d4 Qd6 d5 Qb4+ Qd2 Qc4 b3 Qc5 Ba3 Qb5 Qd4 Qa5+ b4 Qa4 Qc5 Qd7 Nc3 Na6 Qb5 Qxb5 Nxb5 Kd8 c4 Nf6 Nc3 e5 Nf3 Bd6 Ng5 Ke7 Rd1 h5 c5 Bxc5 bxc5 Ke8 Nb5 Rc8 c6 b6 Nxa7 Ra8 Nb5 Rc8 d6 Rf8 d7 Nxd7 cxd7 Kd8 Bxf8 Rb8 Kf2 c6 Nc3 f6 Ne6+ Ke8

Well done! I'm hoping to try Elm out soon as well.