Goal:
  
  A : Type
  B : Type
  a0 : A
  l : list A
  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 : S #|l| = S #|l'|
  ============================
   (mapi
      (λ (i : nat) (_ : B),
         match i with
         | 0 => a0
         | S m => nth m l default
         end) (a :: l') = a0 :: l)


DEBUG:Going to execute:
<coq-core.plugins.ltac::contradiction@0> $1
Level 0: Backtrace:

No primitive equality found.