Collection

    attr a1
    meth init a1 := nil end
    meth put(X) a1:= X|@a1 end
    meth get($) local X in X = @a1.1 a1:= (@a1.2) X end end
    meth isEmpty($) if @a1 == nil then true else false end end
    meth union(C)
       local Y in
      {C isEmpty(Y)}
      if Y == false then
         local X in
        {C get(X)}
        {self put(X)}
        {self union(C)}
         end
      end
       end
    end