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