Three rows separate charge closure from full reaction acceptance. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Opposite charges close
Scaled charge totals start at 0 and end at 0; accepted bit is 1.
0→0,a=1
An electron alone cannot become photons
Scaled charge totals start at -3 and end at 0; accepted bit is 0.
−3→0,a=0
Charge closure is not enough
Scaled charge totals start at 3 and end at 3; accepted bit is 0.