Goal:
  
  A : Type
  H : ReflectEq A
  y : A
  ys : list A
  _x_ : A
  _xs_ : list A
  tl : list A
  ============================
   (ys ++ tl = ys ++ tl)


DEBUG:Going to execute:
solve [ do_atom | use_hyps; do_ccl ]