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
[ trivial
with eq_true
| reflexivity
| symmetry; trivial
| contradiction
| congruence ]