When a right-of-way boat changes course, her obligation to give a keep-clear boat room to keep clear under rule 16.1 begins. The right-of-way boat may give that room by making an additional change of course. If, while the right-of-way boat is making that additional change of course, the keep-clear boat unavoidably breaks a rule of Part 2 Section A, the keep-clear boat is exonerated by rule 43.1(b).