<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def extract_bs(self, cutoff, ligcentroid, resis):
 """Return list of ids from residues belonging to the binding site"""

        return [obres.GetIdx() for obres in resis if self.res_belongs_to_bs(obres, cutoff, ligcentroid)]