To make all public definitions within a namespace available using unqualified identifiers, one can import an entire namespace.
Aliasing is like psuedo subnamespace.
Each time a namepace is closed, all of its imported namespaces are discarded, which means that the next time that namespace is opened, it loses its previous imports. This can be useful for resetting imports, should the need or desire arise.
Since the global namespace cannot be closed, import directives in the global namespace last until the end-of-file.