A captured piece may be placed on the board as your own piece! Note that FEN is incapable of setting up a game mid-swing - no record of captured pieces is possible. If requested, a future version may extend the FEN standard to allow this, particularly if ICC follows the same standard
Get a list of all pieces on the board organized by the type of piece, and the color of the square the piece is on.
Used to determine basic draw conditions
// white pieces
'W' => array('B' => array('W', 'B'), // all bishops
'K' => array('W'),...
// black pieces
'B' => array('Q' => array('B'), // all queens
'K' => array('W'),... // king is on white square