Goal:
  
  A : Type
  B : Type
  C : Type
  R : nat → A → C → Type
  g : A → B → C
  y : B
  ys : list B
  k : nat
  X : All2i (λ (i : nat) (x : A) (y : B), R i x (g x y)) k [] (y :: ys)
  ============================
   (All2i R k [] [])


DEBUG:Going to execute:
apply : eq_sym ; trivial
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.