Also assume user_u, user_r, and user_t are valid user, role, and type identifiers. Determine which of the following security contexts are valid and explain why or why not:
This constraint restricts the ability to "write down," but allows any domain to "write up." Indeed, there is no MLS-related reason to restrict "write up" because it does not constitute a downgrading of information, and there are valid uses of this capability to build MLS-aware security applications. Nonetheless, some MLS system developers like to provide a privilege to control "write up" just like "write down." As an exercise, change the preceding constraint to control writing up and down.