support for content model (<sequence>,<all>,<choice>)
support for top-level <group>
support for <group> 'ref' attribute
support for content model encoding/decoding (incomplete)
support for attribute encoding/decoding (incomplete)
support for <attribute> with inline anonymous <simpleType>
xml tree cleanup (save memory and parsing time)
support for <inclide> and <import>
support for top-level <attribute>
support for <attribute> 'ref' attribute
support for top-level <attributeGroup>
support for <attributeGroup> 'ref' attribute