anchor.diff:                   (valid-move-p move board whitep))
                  +all-moves+)))
   
  -(defun move-origin (final-file final-rank pre-board piece-letter whitep
  +(defun move-origin (final-file final-rank pre-board piece-char whitep
                       capturep &key origin-file origin-rank)
     "Selects from a piece's possible moves the starting location based on a
      destination. The pre-board variable gives the layout before the move."
     (loop for (file-scale rank-scale)
  -     in (piece-moves piece-letter whitep capturep final-rank)
  +     in (piece-moves piece-char whitep capturep final-rank)
        as candidate-file = (+ final-file file-scale)
        as candidate-rank = (+ final-rank rank-scale)
        when (and (in-board-p candidate-file candidate-rank)
                  (eq (get-square pre-board candidate-file candidate-rank)
  -                   (piece-char->symbol piece-letter whitep))
  -               (or (can-jump-p piece-letter)
  +                   (piece-char->symbol piece-char whitep))
  +               (or (can-jump-p piece-char)
                      (not (pieces-between-p pre-board final-file final-rank
                                             candidate-file candidate-rank)))
                  (or (null origin-file) (= origin-file candidate-file))
  
positive.code:               '((0 -1))))))
  
  (defun can-jump-p (piece-char)
    "Determines if a piece can move without concern for pieces on its path"
    (char-equal piece-char #\N))
  
  (defun being-attacked-p (file rank board white-attacker-p)
    "Searches for any pieces that could capture a certain square."
  
negative.code:     (destructuring-bind (origin-file origin-rank)
          (move-origin final-file final-rank board piece whitep capturep
                       :origin-file origin-file :origin-rank origin-rank)
        (move-from-to board origin-file origin-rank final-file final-rank
                      :new-piece (when promotion
                                   (piece-char->symbol promotion whitep)))))
    board)
  