p.boxed
  | Similar to Checkers, with prisoners stacked below capturers.

figure.diagram-container
  img.img-center(src="/variants/Emergo/match_2004_U-con.jpg")
  figcaption.text-center.
    [Wikipedia]
    Game played on an orthogonal depiction of a standard 9x9 Emergo board.

p
  | Nothing disappears in this game: captured pieces become part of the
  | capturing unit. Therefore, a piece or unit will always be understood
  | as a combination of W white (elementary) pieces and B (elementary) black
  | ones, with
  br
  | W >= 0, B >= 0 and W + B >= 1.

p
  | The 9x9 board is initially empty.
  | Each player receives 12 stackable pieces, "in hand".
  | At each turn, a player must pick an action among the followings:
  ul
    li Enter a new piece (W + B = 1) on the board.
    li Move a unit along diagonals, by one square.
    li Capture something.

h3 Entering moves

p.
  Introducing a piece such that the opponent can take it on next turn is
  forbidden, unless another capture is already available.

p At first move, white cannot place a piece at the central point.

p.
  If the opponent already placed all his pieces in hand, then a
  "shadow piece" enters, formed by all the remaining units available
  (thus in this case W + B > 1).

p.
  While you have pieces in hand, none of your pieces can move on the board
  unless they capture something.

h3 Captures

p.
  Captures work exactly as in Checkers: by jumping over a diagonally adjacent
  piece to land on a free square just behind.
  If a capture is possible, then it must be played; in this case no piece can
  be introduced on the board.
  If after a capture another is possible with the same piece, it must also be
  played — except if that implies turning at 180 degrees.

figure.diagram-container
  .diagram.diag12
    | fen:9/9/2a@1a@4/9/4a@4/3D@5/9/5A@3/6b@2 e3:
  .diagram.diag22
    | fen:9/9/2a@1a@4/9/4a@4/3D@5/4ba4/9/9:
  figcaption.
    Before and after black captures, jumping at the marked location.
    The next white piece must be captured too.

p.
  Captures can be described formally as follows.
  If, as white, W1/B1 jumps over W2/B2 at square S2 to land on S1', then
  W1/(B1+1) arrives on S1' while W2/(B2-1) stays on S2.
  If W2 = B2 - 1 = 0, nothing remains at the captured unit location.
  As black: exchange W and B.

p.
  In other words, each unit is a stack of friendly and enemy pieces, with
  friendly pieces on top. After each capture, the prisoners part of the
  stack is incremented, while the jailers quantity at the captured
  location decreases by one.

p.
  When several capturing chains are available,
  the player must select one of the longest (as in Checkers):

figure.diagram-container
  .diagram
    | fen:9/9/2a@1a@4/9/2bb1a@4/3C@5/9/9/9:
  figcaption.
    From https://www.mindsports.nl/:
    White's only option is to capture clockwise.

p.
  A piece can be jumped over several times, as long as it contains
  at least one enemy unit (controling it).

h3 More information

p
  | You are invited to visit the 
  a(href="https://www.mindsports.nl/index.php/arena/emergo/88-rules")
    | authors' website
  | . The rules are also described on 
  a(href="http://www.iggamecenter.com/info/en/emergo.html") iggamecenter
  | , where you can play Emergo.

p Inventors: Christian Freeling and Ed van Zon (1986)