count(a, bag) = 0, a ∈ bag ⊦ a = b
Used simplifier rules: count-del: ⊦ count(a, bag -- b) = count(a, bag) - (a = b ∧ count(a, bag) ≠ 0 ⊃ 1;0);% ⊦ m - 0 = m; (s)