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