* pick and omit don't treat an empty selection correctly (IMHO). specifically:
* pick(d) should probably return nothing
* pick(d, prefix=...) should return only those prefixes
* omit(d) should return d with all keys copied
* omit(d, prefix=...) should return d with all keys that have the specified
prefix removed...
* perhaps use `vars()` during aadict() construction so that it can
handle argparse.Namespace objects ? ...