In every army, the five developments connected with fire must be
known, the movements of the stars calculated, and a watch kept for the
proper days.
Tu Mu’s commentary shows what has to be supplied in order to make
sense out of 以數守之. He says: 須筭星𨇠之數守風起之日乃可發火 “We
must make calculations as to the paths of the stars, and watch for
the days on which wind will rise, before making our attack with
fire.” Chang Yü seems to take 守 in the sense of 防: “We must not
only know how to assail our opponents with fire, but also be on
our guard against similar attacks from them.”