Note the following:
You must define at least one field name for any substructure.
No two fields at the same nesting level can have the same name. Fields at different levels of a structure can have the same name; however, doing so might be questionable programming practice.
You can use the pseudo-name, %FILL, to align fields in a record, and create an unnamed empty field.
You must not include a structure as a substructure of itself, at any level of nesting.