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:
solve
[ do
![ solve
 [ trivial | apply : eq_sym ; trivial ]
 | discriminate
 | contradiction
 | Ltac.split ]
| case not_locked_false_eq_true; Ltac.assumption
| match goal with
  | H:¬ _ |- _ => solve [ case H; trivial ]
  end ]