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


DEBUG:Going to execute:
cbn
DEBUG LTAC Evaluated term: xs