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


DEBUG:Going to execute:
trivial; hnf; intros; (solve
            [ do
            ![ solve
             [ trivial | apply : eq_sym ; trivial ]
             | discriminate
             | contradiction
             | Ltac.split ]
            | case not_locked_false_eq_true; Ltac.assumption
            | match goal with
              | H:¬ _ |- _ => solve [ case H; trivial ]
              end ])