1 INTRODUCTION  
2 DATA BASE  
2.1 Sample and data processing  
2.2 Chopping the seagull  
3 THE STARS IN THE SEAGULL’S WINGS  
4 PHOTOIONIZATION MODELS FOR THE RIGHT WING  
5 UNCERTAINTIES  
6 WHY WE SEE A SEAGULL IN THE BPT DIAGRAM  
7 CONCLUSION