Rule 19.2(a) states, that the right-of-way-boat (Blue) may
choose the side to pass the obstruction (Red). When Blue hails for Room to tack
in time rule 20.2(c) requires Green to give Blue room. If Blue chooses
not to hail and pass astern of Red she shall give Green room to pass between
her and Red as required by rule 19.2(b).