'Returns the index which is preferred to substitute in the final
expression.
The index to substitute is the index with less 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).killable_index
p
>>> KroneckerDelta(p, a).killable_index
p
>>> KroneckerDelta(i, j).killable_index
j
See Also
preferred_index'

@property
def killable_index(self):

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