added lemma

--- a/src/HOL/Data_Structures/Tree2.thy	Thu Sep 26 15:25:51 2019 +0200
+++ b/src/HOL/Data_Structures/Tree2.thy	Thu Sep 26 21:22:58 2019 +0200
@@ -31,4 +31,7 @@
 lemma eq_set_tree_empty[simp]: "set_tree t = {} \<longleftrightarrow> t = Leaf"
 by (cases t) auto
 
+lemma set_inorder[simp]: "set (inorder t) = set_tree t"
+by (induction t) auto
+
 end