'Returns the index which is preferred to keep in the final expression.
The preferred index is the index with more information regarding fermi
level.  If indices contain same information, \'a\' is preferred before
\'b\'.
Examples
>>> from sympy.functions.special.tensor_functions import KroneckerDelta
>>> from sympy import Symbol
>>> a = Symbol(\'a\', above_fermi=True)
>>> i = Symbol(\'i\', below_fermi=True)
>>> j = Symbol(\'j\', below_fermi=True)
>>> p = Symbol(\'p\')
>>> KroneckerDelta(p, i).preferred_index
i
>>> KroneckerDelta(p, a).preferred_index
a
>>> KroneckerDelta(i, j).preferred_index
i
See Also
killable_index'

@property
def preferred_index(self):

    if self._get_preferred_index():
        return self.args[1]
    else:
        return self.args[0]
