Considerable doubt exists as to the precise origin of this custom.对该风俗的确切起源存在着相当大的疑问。Much parliamentary procedure is not written but exists as the custom and practice of Parliament.许多议会流程并不成文，而是作为议会的惯例而存在。Controversy exists as to how safe these drugs are.这些药物的安全程度存在争议。