Please answer the following question: Given the following passage  "Treaties are not necessarily permanently binding upon the signatory parties. As obligations in international law are traditionally viewed as arising only from the consent of states, many treaties expressly allow a state to withdraw as long as it follows certain procedures of notification. For example, the Single Convention on Narcotic Drugs provides that the treaty will terminate if, as a result of denunciations, the number of parties falls below 40. Many treaties expressly forbid withdrawal. Article 56 of the Vienna Convention on the Law of Treaties provides that where a treaty is silent over whether or not it can be denounced there is a rebuttable presumption that it cannot be unilaterally denounced unless:",  answer the following question. Note that the answer is present within the text.  Question: What do some treaties do when they don't allow a state to withdraw?
A:
expressly forbid withdrawal