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


DEBUG:Going to execute:
<coq-core.plugins.ltac::exists@1> $1
DEBUG LTAC Evaluated term: ys