Sequence

attr a1
meth init a1 := nil end
meth isEmpty($) if @a1 == nil then true else false end end
meth first($) case @a1 of H|T then H else nil end end
meth last($)
    local Last in
        fun{Last L}
            case L
            of H|nil then H
            []H|T then {Last T}
            else L
            end
        end
        {Last @a1}
    end
end
meth insertFirst(X) a1 := X|@a1 end
meth insertLast(X) a1 := {Append @a1 [X]} end
meth removeFirst a1 := @a1.2 end
meth removeLast
    local Last in
        fun{Last L}
            case L
            of H|nil then nil
            [] H|T then H|{Last T}
            else nil
            end
        end
        a1 := {Last @a1}
      end
end