interface Session {
   public void submit(Command cmd);
}

interface Queryable {
   public void executeQuery(String query);
}

interface Command {
   public void execute(Queryable queryable);
}

class SessionImpl implements Session, Queryable {
   public void executeQuery(String query) { // execute query in the current session }

   public void submit(Command cmd) {
      cmd.execute(this);
   }
}

class CommandImpl implements Command {
  public void execute(Queryable queryable) {
     queryable.executeQuery("Hello world!");
  }
}