'Returns a sum of NO objects that contain no ambiguous q-operators.
If an index q has range both above and below fermi, the operator F(q)
is ambiguous in the sense that it can be both a q-creator and a q-annihilator.
If q is dummy, it is assumed to be a summation variable and this method
rewrites it into a sum of NO terms with unambiguous operators:
{Fd(p)*F(q)} = {Fd(a)*F(b)} + {Fd(a)*F(i)} + {Fd(j)*F(b)} -{F(i)*Fd(j)}
where a,b are above and i,j are below fermi level.'

def _expand_operators(self):

    return NO(self._remove_brackets)
