Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  l : list A
  r2 : list B
  h : All2 R l ([] ++ r2)
  ============================
   (∑ l1 l2 : list A, l = l1 ++ l2 × All2 R l1 [] × All2 R l2 r2)


DEBUG:Going to execute:
exists [],l
DEBUG LTAC Evaluated term: r1