If a differential archetype contains text of the form:
The proper implication of the overlay should be that it is a replacement from items down, i.e. that the result looks like the child archetype block.
If instead of 'items' it had '/items' then a proper overlay of the child structure on a copy of the parent structure should occur.
Currently the flattener does the latter kind of overlay even with 'items' as the attribute name.