'Is this FermionicOperator restricted with respect to fermi level?
Return values:
1  : restricted to orbits above fermi
0  : no restriction
-1 : restricted to orbits below fermi
>>> from sympy import Symbol
>>> from sympy.physics.secondquant import F, Fd
>>> a = Symbol(\'a\', above_fermi=True)
>>> i = Symbol(\'i\', below_fermi=True)
>>> p = Symbol(\'p\')
>>> F(a).is_restricted
1
>>> Fd(a).is_restricted
1
>>> F(i).is_restricted
-1
>>> Fd(i).is_restricted
-1
>>> F(p).is_restricted
0
>>> Fd(p).is_restricted
0'

@property
def is_restricted(self):

    ass = self.args[0].assumptions0
    if ass.get('below_fermi'):
        return (-1)
    if ass.get('above_fermi'):
        return 1
    return 0
