⊦ bag0 ⊆ bag1 ↔ (∀ a. count(a, bag0) ≤ count(a, bag1))
⊦ count(a, bag1) ≠ 0 → ((∀ a0. count(a0, bag0) ≤ count(a0, bag1 -- a)) ↔ bag0 ++ a ⊆ bag1)
Used terms: bag1 -- a, bag0