Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → C → Type
  c : C
  a : A
  ls2 : list A
  X0 : All2 (λ (x : A) (y : B), P x y c) (a :: ls2) []
  X1 : All (λ z : C, All2 (λ (x : A) (y : B), P x y z) (a :: ls2) []) []
  ============================
   (All2 (λ (x : A) (y : B), All (P x y) [c]) (a :: ls2) [])


DEBUG:Going to execute:
lazymatch x with
           | [] => idtac
           | _ :: _ => idtac
           end