ORA-DM-MAPPINGS-NIL

*Data Model, Section 4.3.3 PSVI to data model mapping*
The mapping to the nilled attribute seems to assume that only 
xsi:nil='true' is permitted by XML Schema Part 1.
XML Schema Part 1, section 3.2.7, second box, says that the xsi:nil 
attribute is type boolean, meaning that its value may be 'false'. Thus 
the mere presence of an xsi:nil attribute does not insure that the 
element has been nilled; you also need to look at the attribute value.