⊦ count(a, bag0) - (a = b ∧ count(b, bag0) ≠ 0 ⊃ 1;0) = 0, a ∈ bag0
Used simplifier rules: count-del: ⊦ count(a, bag -- b) = count(a, bag) - (a = b ∧ count(a, bag) ≠ 0 ⊃ 1;0);