Goal:
  
  A : Type
  H : ReflectEq A
  x : A
  xs : list A
  y : A
  ys : list A
  tl : list A
  ============================
   (∃ l' : list A, xs ++ tl = xs ++ l')


DEBUG:Going to execute:
apply : eq_sym ; trivial
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.