Section BasicProperties.
Variable s s' s'' s1 s2 s3 : t.
Variable x x' : elt.

(* Specifications: *)
Lemma equal_refl : s[=]s.
Lemma elements_empty : elements empty = nil.
Definition of_list (l : list elt) := List.fold_right add empty l.
Lemma equal_refl : s[=]s.
Lemma elements_empty : elements empty = nil.
Definition of_list (l : list elt) := List.fold_right add empty l.