sub visit_breadth_first {
   my ($node, $visitor) = @_;
   # Initial value:
   my @to_visit = ($node);
   # Loop as long as @to_visit isn't empty:
   while (@to_visit) {
      # Remove an element:
      my $current = shift(@to_visit);
      # Maybe add some elements:
      push(@to_visit, @{$current->children()});
      &$visitor($current);
   }
}