Goal:
  
  X : Type
  Y : Type
  f : ∀ x : X, In x [] → Y
  g : ∀ x : X, In x [] → Y
  all_eq : ∀ (x : X) (isin : In x []), f x isin = g x isin
  ============================
   (map_In [] f = map_In [] g)


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"