We will distiguish here between Simple and Composite enclosures. Simple enclosure will define a single external element, while Composite enclosure will have an internal structure and will be capable of containing other Simple or Composite elements.
An RSS ``item'' or ATOM ``entry'' element could contain multiple Simple or Composite enclosure elements. If more than one such element is included under either ``item'' or ``atom:entry'', then they are considered to be part of an implied Composite enclosure with kind ``mixed'' (see section 3.3).
All enclosure elements are defined in separate namespace. We will use 2 namepsaces: one for RSS 2.0 enclosure syntax and another for for RSS 1.0. Respective namsepace defintions are:
xmlns:enc="http://crocodile.org/ns/rss/2.0/enclosures" xmlns:enc1="http://crocodile.org/ns/rss/1.0/enclosures"