sub hash_filter {
    return {
        map {
            ref $_[1]->{$_} eq 'HASH'
            ? ref $_[0]->{$_} eq 'HASH'
              ? ($_, hash_filter( $_[0]->{$_}, $_[1]->{$_} ))
              : croak "bad filter: on '$_', expected HASH\n"
            : ($_, $_[0]->{$_})
        } grep { exists $_[0]->{$_} } keys %{$_[1]}
    }
}