  This paper presents a coding scheme for an insertion deletion substitution
channel. We extend a previous scheme for the deletion channel where polar codes
are modified by adding "guard bands" between segments. In the new scheme, each
guard band is comprised of a middle segment of '1' symbols, and left and right
segments of '0' symbols. Our coding scheme allows for a regular hidden-Markov
input distribution, and achieves the information rate between the input and
corresponding output of such a distribution. Thus, we prove that our scheme can
be used to efficiently achieve the capacity of the channel. The probability of
error of our scheme decays exponentially in the cube-root of the block length.
