∀ {α : Type u_1} {C : Multiset α → Sort u_3} {C_0 : C 0} {C_cons : (a : α) → (m : Multiset α) → C m → C (a ::ₘ m)}
  {C_cons_heq :
    ∀ (a a' : α) (m : Multiset α) (b : C m), C_cons a (a' ::ₘ m) (C_cons a' m b) ≍ C_cons a' (a ::ₘ m) (C_cons a m b)}
  (a : α) (m : Multiset α), (a ::ₘ m).recOn C_0 C_cons C_cons_heq = C_cons a m (m.recOn C_0 C_cons C_cons_heq)