Goal:
  
  A : Type
  B : Type
  P : B → A → A → Prop
  h : ∀ b : B, Symmetric (P b)
  x : B
  y : A
  z : A
  l : list B
  l' : list A
  l'' : list A
  H : P x y z
  hl : Forall3 P l l' l''
  IHhl : Forall3 P l l'' l'
  ============================
   (P x y z)


DEBUG:Going to execute:
let do_atom := solve
           [ trivial
           with eq_true
           | reflexivity
           | symmetry; trivial
           | contradiction
           | congruence ]
           in
           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"