The ParentDocumentBead class looks up the parent
chain to find a parent that was written in MXML.
Because it is usually rare for an application
to need to know this information, an optional bead
is used to compute it, instead of baking in the
overhead of a recursive infrastucture to store
this information. It is intended to be used
as a bead in the top-level tag of an MXML document.