'Matches any HDU that is not recognized as having either the SIMPLE or
XTENSION keyword in its header\'s first card, but is nonetheless not
corrupted.
TODO: Maybe it would make more sense to use _NonstandardHDU in this
case?  Not sure...'

@classmethod
def match_header(cls, header):

    return (first(header.keys()) not in ('SIMPLE', 'XTENSION'))
