sub range_query {
    my ($node, $from, $to) = @_;
    return () unless $node;
    return range_query ($node -> left,  $from, $to) if $to   < $node -> key;
    return range_query ($node -> right, $from, $to) if $from > $node -> key;
    return range_query ($node -> left,  $from, $to), $node -> key,
           range_query ($node -> right, $from, $to)
}