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


DEBUG:Going to execute:
(solve [ do_atom | use_hyps; do_ccl ]) ||
             fail "Cannot solve this goal"