mwg API
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    Interface ChessState

    interface ChessState {
        board: (ChessPiece | null)[];
        castling: ChessCastling;
        enPassant: number | null;
        turn: ChessSide;
    }
    Index
    board: (ChessPiece | null)[]
    castling: ChessCastling
    enPassant: number | null

    a square a pawn could capture onto, left behind by a double push

    turn: ChessSide