structQueue{privatevarelements:[T]=[]publicmutatingfuncenqueue(_element:T){elements.append(element)}publicmutatingfuncdequeue()->T?{returnelements.popFirst()//ERROR!}publicmutatingfuncdequeue2()->T?{returnelements.removeFirst()}}我为popFirst得到的错误是:cannotusemutatingmemberonimmutablevalue:'self'isim