Definition is_chain (F : part_set (part_set X)) :=
  forall A B : part_set X,
  in_part A F -> in_part B F -> included A B \/ included B A.(** - Quoting the Maximal Principle from page 50:

 Let $\cal F$ be a family of sets. If for each chain $\cal C\subset \cal F$ there
 exists a member of $\cal F$ that contains each member of $\cal C$, then $\cal F$
 contains a maximal element.*)

(** %\tocdef{MAXIMAL\_PRINCIPLE}% *)
Axiom
  MAXIMAL_PRINCIPLE :
    forall F : part_set (part_set X),
    (forall C : part_set (part_set X),
     is_chain C ->
     included C F ->
     exists A : part_set X,
       in_part A F /\ (forall B : part_set X, in_part B C -> included B A)) ->
    exists A : part_set X, maximal F A.

Lemma every_vecsp_has_a_basis : forall (F : field) (V : vectorspace F), exists beta : part_set V, is_basis beta.