Fix (trivial case of) non-membership check.

def fix_e713(self, result):
        """Fix (trivial case of) non-membership check."""
        (line_index, offset, target) = get_index_offset_contents(result,
                                                                 self.source)

        # to convert once 'not in' -> 'in'
        before_target = target[:offset]
        target = target[offset:]
        match_notin = COMPARE_NEGATIVE_REGEX_THROUGH.search(target)
        notin_pos_start, notin_pos_end = 0, 0
        if match_notin:
            notin_pos_start = match_notin.start(1)
            notin_pos_end = match_notin.end()
            target = '{}{} {}'.format(
                target[:notin_pos_start], 'in', target[notin_pos_end:])

        # fix 'not in'
        match = COMPARE_NEGATIVE_REGEX.search(target)
        if match:
            if match.group(3) == 'in':
                pos_start = match.start(1)
                new_target = '{5}{0}{1} {2} {3} {4}'.format(
                    target[:pos_start], match.group(2), match.group(1),
                    match.group(3), target[match.end():], before_target)
                if match_notin:
                    # revert 'in' -> 'not in'
                    pos_start = notin_pos_start + offset
                    pos_end = notin_pos_end + offset - 4     # len('not ')
                    new_target = '{}{} {}'.format(
                        new_target[:pos_start], 'not in', new_target[pos_end:])
                self.source[line_index] = new_target