⊦ a ∈ bag → (∃ bag0. bag = bag0 ++ a)
bag0 ++ a ⊆ bag1, a ∈ bag1 → (∃ bag0. bag1 = bag0 ++ a) ⊦ ∃ bag2. bag1 = bag2 ++ a ∧ bag0 ⊆ bag2
Used terms: a, bag1