sub add_line {
   my ($tree,@line) = @_;
   my $first_element = shift @line;
   if (is_a_folder_title($first_element)) { # TODO: implement is_a_folder_title()
     $tree->{$first_element} ||= {}; # create empty hashref if none exists, yet
     add_line($tree->{$first_element},@line);
   }
   else {
     $tree = [ $first_element, @line ];
   }
}