TSO/E provides the ALTLIB command, which lets users define alternative libraries of REXX execs and CLISTs for implicit execution. TSO/E also provides initialization and termination exits that you can use to customize the ALTLIB command. The initialization exit can restrict users from using the ALTLIB command, can change operands supplied by the user, or pass the address of a new command buffer. The termination exit can perform clean-up processing, such as releasing storage obtained by the initialization exit. For more information, see Customizing the ALTLIB command.