Goal:
  
  A : Type
  B : Type
  a : B
  l' : list B
  default : A
  IHl' : ∀ l : list A,
           #|l| = #|l'| → mapi (λ (i : nat) (_ : B), nth i l default) l' = l
  H : 0 = S #|l'|
  ============================
   (mapi (λ (i : nat) (_ : B), match i with
                               | 0 | _ => default
                               end) (a :: l') = [])


DEBUG:Going to execute:
apply : eq_sym ; trivial
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.