Goal:
  
  A : Type
  B : Type
  R : A → B → Prop
  a : A
  l : list A
  H : ∀ (x : A) (y : B), R x y
  IHl : ∀ l' : list B, #|l| = #|l'| → Forall2 R l l'
  b : B
  l0 : list B
  e : S #|l| = #|b :: l0|
  ============================
   (R a b)


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"