Notation Power := (Pow A leA).Notation Lex_Exp := (lex_exp A leA).Notation ltl := (Ltl A leA).Notation Descl := (Desc A leA).Notation List := (list A).Notation "<< x , y >>" := (exist Descl x y) (at level 0, x, y at level 100).

Lemma desc_tail : forall (x : List) (a b : A), Descl (b :: x ++ [a]) -> clos_refl_trans A leA a b.