⊦ count(b, bag -- a) - (count(b, bag -- a) = 0 ⊃ 0;1) = count(b, bag -- b -- a)
Used simplifier rules: count-del-same: ⊦ count(a, bag -- a) = count(a, bag) - (count(a, bag) = 0 ⊃ 0;1);