Goal:
  
  A : Type
  B : Type
  P : B → A → A → Type
  Q : B → A → A → Type
  b : B
  bs : list B
  hd : A
  hd' : A
  tl : list A
  p : P b hd hd'
  e : #|bs| = #|tl|
  HPQ : ∀ (i : B) (x y : A), P i x y → ∑ z : A, Q i x z × Q i y z
  x : A
  q : Q b hd x
  q0 : Q b hd' x
  ============================
   (#|bs| = #|tl|)


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"