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