add Int_atMost

--- a/src/HOL/Set_Interval.thy	Thu Dec 06 23:07:10 2012 +0100
+++ b/src/HOL/Set_Interval.thy	Fri Dec 07 14:28:57 2012 +0100
@@ -351,6 +351,9 @@
 lemma Int_greaterThanLessThan[simp]: "{a<..<b} Int {c<..<d} = {max a c <..< min b d}"
 by auto
 
+lemma Int_atMost[simp]: "{..a} \<inter> {..b} = {.. min a b}"
+  by (auto simp: min_def)
+
 end