Manually reverted commits that caused Sequence and Tuple to return a non-None default (primarily 67d3da8). This also gets rid of LeafAttribute than didn't seem to be doing anything useful anyway. The default should be None unless the user explicitly sets it to something. Otherwise, it makes it look there was data present, e.g. None vs (None, None) for a 2-value Tuple. Reinstated unexpected keyword check that seems to have been deleted by commit aa5bb0e.
… full validatish exception