namespace handling models

am wondering if anyone can think of any 
other models for how xml applications handle 
namespaces that are not in the applications 
known namespaces than these:

parse-tree: all elements in the unknown 
namespace is ignored, and children of the 
element are parsed.

ignore-tree: as soon as an element in an 
unknown namespace is encountered that 
element and any children are ignored.

fail: unknown namespaces are a cause for 
failure. Generally due to validation 
failure.