Goal:
  
  A : Type
  B : Type
  f : nat → A
  a : B
  l : list B
  b : 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'
  eq : #|l| = #|l'|
  ============================
   (#|l| = #|l'|)


DEBUG:Going to execute:
let rec do_ccl :=
            try do_atom; repeat (do_intro; try do_atom); try arith_congr;
             (solve [ split; do_ccl ])
           in
           (solve [ do_atom | use_hyps; do_ccl ]) ||
             fail "Cannot solve this goal"