| ∀ b. count(b, bag1) = (a = b ⊃ 1;0) + count(b, bag0) ⊦ bag1 = bag0 ++ a |
⊦ bag0 = bag1 ↔ (∀ a. count(a, bag0) = count(a, bag1))
∀ b. count(b, bag1) = (a = b ⊃ 1;0) + count(b, bag0) ⊦ ∀ a0. count(a0, bag1) = count(a0, bag0 ++ a)
Used terms: bag0 ++ a, bag1