'Is the index of this FermionicOperator restricted to values above 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_only_above_fermi
True
>>> F(i).is_only_above_fermi
False
>>> F(p).is_only_above_fermi
False
The same applies to creation operators Fd'

@property
def is_only_above_fermi(self):

    return (self.is_above_fermi and (not self.is_below_fermi))
