Goal:
    A : Type  B : Type  C : Type  P : A → B → C → Type  a : A  ls2 : list A
  ============================
   (([] = []) + All2 (λ (x : A) (y : B), All (P x y) []) (a :: ls2) [])%type


DEBUG:Going to execute:
progress subst
Level 0: Backtrace:

Failed to progress.
Level 0: Backtrace:

Failed to progress.