Lesson is that a starboard tack boat can also be an obstruction.
True! A boat you are required to keep clear of is an obstruction to you. If she is also an obstruction to a boat overlapped on the inside, then rule 19 may require you give her room to avoid her.
I encourage everyone to flip to the back of their rule books and read the Definitions. They are three of the most illuminating pages in the book.
Regards,
Eric