Goal:
    A : Type  B : Type  P : list A → list A → A → A → B → Type
  ============================
   (∀ ls2 ls3 : list A,
      [] ≠ [] ∨ #|ls2| = #|ls3|
      → All
          (λ z : B,
             All2_fold (λ (l1 l2 : list A) (x y : A), P l1 l2 x y z) ls2 ls3)
          []
        → All2_fold (λ (l1 l2 : list A) (x y : A), All (P l1 l2 x y) []) ls2
            ls3)


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrmove@1> $1 $2