⊦ bag0 = bag1 ↔ (∀ a. count(a, bag0) = count(a, bag1))
∀ a0. count(a0, bag0 ++ a) = count(a0, bag1 ++ a) ⊦ count(a0, bag0) = count(a0, bag1)
Used terms: bag1 ++ a, bag0 ++ a