The main line continues with Black playing at a. In 1964, Hashimoto Utaro played b instead, and so far this seems to be valid as well. With c, this transposes to the one-space low pincer.
The classical response is at a. On the other hand, b is an innovation by Kitani Minoru and may be discussed here. After a, Black may continue with c or d.
After , Black may exchange a for b before making an extension along the top. However, White may choose not to answer a and play elsewhere instead. Extending immediately leaves the aji of White c, however, possibly taking the corner.