Goal:
  
  A : Type
  B : Type
  f : nat → A
  b : B
  l' : list B
  H : #|[]| = #|b :: l'|
  ============================
   (mapi (λ (i : nat) (_ : B), f i) [] =
    mapi (λ (i : nat) (_ : B), f i) (b :: l'))


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

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.