Goal:
  
  X : Type
  Y : Type
  R : nat → X → Y → Type
  n : nat
  xs' : list X
  l : list Y
  a : All2i R n ([] ++ xs') l
  ============================
   (All2i R n [] ?Goal0 × All2i R n xs' ?Goal1)


DEBUG:Going to execute:
split; eauto; constructor