Instance EqDec_share : EqDec t := EqDec_canonTree.(* Credits for the next part of this file:
  Specification of "unrel" operator by Andrew W. Appel and Robert Dockins
  Definition of "unrel", and all the proofs about it, by Le Xuan Bach
*)

   Program Definition tree_decompose (ct : canonTree) :(canonTree * canonTree) :=
     let (t, pf) := ct in
     match t with
     |Leaf b => (Leaf b, Leaf b)
     |Node t1 t2 => (t1 ,t2)
     end.Instance decompose_tree : decomposible t :=
     Decomposible tree_decompose.Fixpoint tree_heightP (t : ShareTree) : nat :=
   match t with
   | Leaf b  => 0
   | Node l r => (max (tree_heightP l) (tree_heightP r)) + 1
  end.Definition tree_height (t : canonTree) := tree_heightP (proj1_sig t).Definition tree_height_zero (t : canonTree) : {tree_height t = 0} + {tree_height t <> 0}.Instance tree_heightable : heightable t :=
    Heightable tree_height tree_height_zero.Function unrel (t1 : t) (t2 : t) {measure tree_height t1} : t :=
   match t1 with
    | exist _ (Leaf b) _ => t2
    | _ => let (ltr1, rtr1) := decompose t1 in
           let (ltr2, rtr2) := decompose t2 in
              match ltr1 with
               | exist _ (Leaf true) _ => ltr2
               | exist _ (Leaf false) _ => unrel rtr1 rtr2
               | _ => unrel ltr1 ltr2
              end
   end.

Lemma unrel_rel: forall x sh, nonidentity x -> unrel x (rel x sh) = sh.