(self)
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
Returns the index which is preferred to substitute in the final
        expression.
Returns the index which is preferred to substitute in the final expression .
def killable_index(self):
        """
        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

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