private void ensureUnreachableIsMutable() {
      if (!unreachable_.isModifiable()) {
        unreachable_ = new com.google.protobuf.LazyStringArrayList(unreachable_);
      }
      bitField0_ |= 0x00000004;
    }
<pre>
A token identifying a page of results the server should return.
</pre>

<code>string next_page_token = 2;</code>

@param value The bytes for nextPageToken to set.
@return This builder for chaining.
java-visionai/proto-google-cloud-visionai-v1/src/main/java/com/google/cloud/visionai/v1/ListSeriesResponse.java