'Does the index of this FermionicOperator allow values below fermi?
>>> from sympy import Symbol
>>> from sympy.physics.secondquant import F
>>> a = Symbol(\'a\', above_fermi=True)
>>> i = Symbol(\'i\', below_fermi=True)
>>> p = Symbol(\'p\')
>>> F(a).is_below_fermi
False
>>> F(i).is_below_fermi
True
>>> F(p).is_below_fermi
True
The same applies to creation operators Fd'

@property
def is_below_fermi(self):

    return (not self.args[0].assumptions0.get('above_fermi'))
