A namespace looks like abc::def::ghi.
Namespaces are more subtle than one would naively assume. For namespaces to be genuinely extensible, they cannot know anything beyond their scope. This means that they don't have an absolute index. They can be positioned relative to another namespace only by a 'sliding' match. This matching can potentially be ambiguous, so if multiple matches occur an error is returned. Namespaces can never be empty.