<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def aos_as_list(self):
        '''
        Returns a list of atomic orbitals, sorted from lowest to highest energy
        '''
        return sorted(chain.from_iterable(
            [self.aos[el] * int(self.composition[el]) for el in self.elements]
        ), key=lambda x: x[2])