sub path_to_op {
  my ($node,$tgt,$path)=@_;
  $path||='';
  return $path if $node==$tgt;
  
  foreach my $key (keys %$node) {
     my $path=path_to_op($node->{$key},$tgt,"$path{$key}");
     return $path if $path;    
  }
  return ''
}