import LinkedList in {

  contract makeGameBoard(return) {
    new players in {

      players!([]) |

      contract registerPlayer(endorse) {
        new friends, getFriends in {

          friends!([]) |

          contract endorse(@subject, @level, return) {
            for (oldFriends <- friends) {
              LinkedList!("prepend", [[subject, level], *oldFriends], *friends)
            }
          } |

          contract getFriends(@level, return) {
            new filter in {
              contract filter(@friend, @acc, return) = {
                match friend {
                  [s, l] => {
                    match l == level {
                      true => {
                        LinkedList!("prepend", [s, acc], return)
                      }
                      _ => { return!(acc) }
                    }
                  }
                  _ => { return!(acc) }
                }
              } |
              for (f <- friends) {
                LinkedList!("fold", [*f, [], *filter], return) |
                friends!(f)
              }
            }
          } |

          for (oldPlayers <- players) {
            LinkedList!("prepend", [*getFriends, *oldPlayers], *players)
          }

        }
      } |

      contract getPlayers(return) {
        new gather in {
          contract gather(@player, @acc, return = {
            LinkedList!("prepend", [player, acc], return)
          } |
          for (p <- players) {
            LinkedList!("fold", [*p, [], *gather], return)
            players!(p)
          }
        }
      }

      contract getCerts(seed, level, return) {
         // ouch.
      }
    }
  }
}