Goal:
  
  A : Type
  P : A → Type
  x : A
  l : list A
  p : P x
  X : All P l
  IHX : In x l → ∥ P x ∥
  ============================
   (P x)


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"