[modeller_usage] selection of loop

Dear everyone, i have a question and i need your help. i want to refine loops (50-60 and 90-100) of my model after building model by modeller9.14. however, i don't know how to set the two regions. the part codes of my script is:

return selection(self.residue_range('50:', '60:'), self.residue_range('90:', '100:'), selection_status='INITIALIZE')

is right or not? And what's the difference between self.residue_range('90:', '100:') and self.residue_range('90', '100')? thank you very much! ai yixin