sub FIRSTKEY {
    my $self = shift;
    if (! @_) {
        return $self->[0]->{FIRSTKEY};
    }
    # ... walk the tree to gather nested keys
    $self->[1]->{iter} = \@list;
    # $_[0] is an empty array provided by the machinery that calls ->FIRSTKEY underneath.
    my $name = shift(@list);
    push(@{$_[0]}, $name); # Either way works...
    return $name;
}