Cell

type UserId  = user_id(Int);
type GroupId = group_id(Int);

schema OnlineForum {
  next_id : Int = 0;
  user(UserId)
    username       : String [unique],
    signup_date    : Date,
    first_name     : String,
    last_name      : String,
    date_of_birth? : Date;
  chat_group(GroupId)
    name  : String,
    admin : UserId;
  admin(_, id) -> user(id);
  member(UserId, GroupId)
    joined_on : Date,
    karma     : Int;
  member(u, g) -> user(u), chat_group(g);
  friends(UserId | UserId)
    since : Time;
  friends(u1, u2) -> user(u1), user(u2);
}

using OnlineForum {
  String full_name(UserId u) = first_name(u) & " " & last_name(u);
  [UserId] users_who_signed_up_on(Date d) = [u : u <- signup_date(?, d)];
}