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


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 ]