of the few i tried, i can't find a single inbuilt identifier that treats $ident( [ ... , ] 0) and $ident( [ ... , ] 0).prop any differently (when the prop doesn't change the nature of the collection), even when the prop is invalid! if the prop is such that it only becomes relevant when applied to an individual item in the collection you're dealing with, you could probably understand why (and, from a coder's perspective, how) this result comes about. it just seems much more sensible than returning $null.
