Class ArchiveNotRegistered

              object --+        
                       |        
exceptions.BaseException --+    
                           |    
        exceptions.Exception --+
                               |
                              ArchiveNotRegistered

Tried to access an unregistered archive.

Namespace existence predicates, implementing `NamespaceObject.exists`,
return a boolean value for the existence of a name in the Arch namespace.
When the archive a name belongs to is not registered, existence cannot be
decided, so this exception is raised.

x.__init__(...) initializes x; see x.__class__.__doc__ for signature