'Comparison between L{FirstError} and other L{FirstError} instances
is defined as the comparison of the index and sub-failure of each
instance.  L{FirstError} instances don\'t compare equal to anything
that isn\'t a L{FirstError} instance.
@since: 8.2'

def __cmp__(self, other):

    if isinstance(other, FirstError):
        return cmp((self.index, self.subFailure), (other.index, other.subFailure))
    return (-1)
