fact: Fixpoint fin_add_inv {n1 n2} : ∀ (P : fin (n1 + n2) → Type) (H1 : ∀ i1 : fin n1, P (Fin.L n2 i1)) (H2 : ∀ i2, P (Fin.R n1 i2)) (i : fin (n1 + n2)), P i := match n1 with | 0 => λ P H1 H2 i, H2 i | S n => λ P H1 H2, fin_S_inv P (H1 0%fin) (fin_add_inv _ (λ i, H1 (FS i)) H2) end.
imports: stdpp(options)