Since everyone knows the basics we can get into the various stages 
starting with the XML handling.

XML Input

FOP can take the input XML in a number of ways:
- SAX Events through SAX Handler
- DOM which is converted into SAX Events
- data source which is parsed and converted into SAX Events
- XML+XSLT which is transformed using an XSLT Processor and the result is
fired as SAX Events

The SAX Events which are fired on the SAX Handler, class FOTreeBuilder,
must represent an XSL:FO document. If not there will be an error. Any
problems with the XML being well formed are handled here.

Element Mappings

The element mappings is a hashmap of all the elements in a particular
namespace. This makes it easy to create a different object for each
element. Element mappings are static to save on memory.

To add an extension a developer can put a jar in the classpath that
contains the file "/META-INF/services/".
This must contain a line with the fully qualified name of a class that
implements the "" interface. This will
then be loaded automatically at the start.
Internal mappings are: FO, SVG and Extension (pdf bookmarks)

Tree Building

The SAX Events will fire all the information for the document with start
element, end element, text data etc. This information is used to build up
a representation of the FO document. To do this for a namespace there is a
set of element mappings. When an element + namepsace mapping is found then
it can create an object for that element. If the element is not found then 
it creates a dummy object or a generic DOM for unknown namespaces.

The object is then setup and then given attributes for the element. For
the FO Tree the attributes are converted into properties. The FO objects 
use a property list mapping to convert the attributes into a list of 
properties for the element.
For other XML, for example SVG, a DOM of the XML is constructed. This DOM
can then be passed through to the renderer.
Other element mappings can be used in different ways for example to create 
elements that create areas during the layout process or Setup information 
for the renderer etc.

While the tree building is mainly about creating the FO Tree there are 
some stages that can propagate to the renderer. At the end of a page 
sequence we know that all pages in the page sequence can be rendered 
without being effected by any further XML. The end of the XML document 
also tells us that we can finalise the output document.

Associated Tasks

Error handling for xml not well formed.
Error handling for other XML parsing errors.
Developer info for adding namespace handlers.

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