(** [] *)

(** What went wrong?  Clearly, [elements_relate] is true; you just
    explained why.  And clearly, it's not true, because
    [not_elements_relate] is provable in Coq.  The problem is that the
    tree [(T (T E 3 v E) 2 v E)] is bogus: it's not a well-formed
    binary search tree, because there's a 3 in the left subtree of the
    2 node, and 3 is not less than 2.

    If you wrote a good answer to the [elements_relate_informal]
    exercise, (that is, an answer that is only subtly wrong), then the
    subtlety is that you assumed that the search tree is well formed.
    That's a reasonable assumption; but we will have to prove that all
    the trees we operate on will be well formed.  *)

(* ################################################################# *)
(** * Representation Invariants *)

(** A [tree] has the [SearchTree] property if, at any node with key
     [k], all the keys in the left subtree are less than [k], and all
     the keys in the right subtree are greater than [k].  It's not
     completely obvious how to formalize that!  Here's one way: it's
     correct, but not very practical. *)

Fixpoint forall_nodes (t: tree) (P: tree->key->V->tree->Prop) : Prop :=
 match t with
 | E => True
 | T l k v r => P l k v r /\ forall_nodes l P /\ forall_nodes r P
 end.Definition SearchTreeX (t: tree) :=
 forall_nodes t
   (fun l k v r => 
      forall_nodes l (fun _ j _ _ => j<k) /\
      forall_nodes r (fun _ j _ _ => j>k)).

Lemma example_SearchTree_bad: forall v, ~SearchTreeX (T (T E 3 v E) 2 v E).