The fragment identifier interface is implemented by any class that
is to be used by a SAX content handler to identify fragments.
Usage of fragment identifiers requires:
1. implementing the initialisation of the identifier - including
setting up any properties specific to the type of identifier being
2. implementing the adjustment of the state of the fragment identifier
whenever the end of an element is encountered by the SAX parser.
3. implementing the fragment recognition code.
Fragment identifiers are instantiated and initialised by the
documentStart event handler in the SAX content handler.
Fragment identification and state updating is triggered within the
elementStart event handler.
Fragment state updating is triggered within the elementEnd event handler.
Determines if the element starts a new fragment and if so:
creates the fragment
sets the fragment index
stores any ID attributes on the fragment root in the fragment metadata
and in the element state object
adds the fragment to the loader