A component that supports this service preserves XML attributes,
unknown by its parser, that belong to the XML element representing
it (the component).
This container holds the AttributeData elements
that represent uninterpreted XML attributes of a paragraph.
The idea behind this property is that a parser can stow
away all attributes that it cannot handle by itself on reading
an XML file. When the file is stored again, the unknown
attributes can be written back without loss.
