package Tree;

sub next_inorder{
                my $self = persist();
                yield { $self->{left}->next_inorder } if $self->{left};
                yield { $self };
                yield { $self->{right}->next_inorder if } $self->{right};
                yield_return undef;
}


# and later...
while (my $node = $root->next_inorder()) {
         print $node->{data};
}