Increments the index by a natural number.
Decrements the index by a natural number.
(sequentIndex: any2stringadd[SequentIndex]).+(other)
(sequentIndex: Ordered[SequentIndex]).<(that)
(sequentIndex: Ordered[SequentIndex]).<=(that)
(sequentIndex: Ordered[SequentIndex]).>(that)
(sequentIndex: Ordered[SequentIndex]).>=(that)
(sequentIndex: Ordered[SequentIndex]).compare(that)
(sequentIndex: Ordered[SequentIndex]).compareTo(that)