bag0 ⊆ bag ⊦ bag0 ⊆ bag ++ a
Used simplifier rules: subset-plus: ⊦ ¬ ¬ bag0 ++ a ⊆ bag1 ↔ (∃ bag2. bag1 = bag2 ++ a ∧ bag0 ⊆ bag2);