sub filter::can_connect_to {
    my ($self,$input) = @_;
    return 1;
  };

  sub filter::connect_output {
    my ($self,$next_filter) = @_;
    # ... do magic
  };

  sub filter::on_input_connect {
    my ($self,$input) = @_;
  };

  sub filter::process {
    my ($self, $data) = @_;
    # ... do magic
  };

  sub filter::describe {
    return "Magic filter";  
  };