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