BlockArgument& block_arg) {
  // check that inputs length is same as num_replicas.
  if (inputs.size() != replicate.getN()) {
    return replicate.emitError()
           << "Expected numper of inputs (" << inputs.size()
           << ") to append to replicate to be num_replicas ("
           << replicate.getN() << ")";
  }

  // add block arg to region. This is in $body.