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