code: @Override
    public CompletableFuture<Void> mutate(final Mutation mutation) {
      return toCompletableFuture(helper.mutate(mutation))
          .thenApply(r -> null);
    }
docs: {@inheritDoc}
  
  Being a Mutation. This method will block if either of the following are true:
  1) There are more than {@code maxInflightRpcs} RPCs in flight
  2) There are more than {@link #getWriteBufferSize()} bytes pending