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:
<coq-core.plugins.ssreflect::ssrtcldo@0> arg