Goal:
  
  X : Type
  Y : Type
  R : nat → X → Y → Type
  n : nat
  l : list X
  ys' : list Y
  a : All2i R n l ([] ++ ys')
  ============================
   (All2i R n [] [] × All2i R (n + #|[]|) l ys')


DEBUG:Going to execute:
cbn