2.1. xxx
Bob: At first the Chin/Chen derivation seemed to work only if I would
arbitrarily change forces into accelerations. I thought that I might
have made a mistake in my implementation, since I had only tested it
for equal masses, but when I tested it for the Pythagorean problem, my
old implementation was fourth-order, and the newly derived
implementation was only second order. I was baffled.