attr a1
meth init a1 := nil end
meth isEmpty($)
    if @a1 == nil then true
    else
        false
    end
end
meth first($)
    case @a1 of nil then nil
    [] 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