Incorrect FEN generated:

[xd009642]

For the game sequence:

1. d4 f6
2. d5 e5

The Board::to_string() implementation generates the following FEN string: "rnbqkbnr/pppp2pp/5p2/3Pp3/8/8/PPP1PPPP/RNBQKBNR w KQkq e5 0 1".

This is incorrect, the en-passant square should be e6.

[mariogeiger]

I realized the same issue