proper ceil operation;

--- a/src/Pure/General/rat.ML	Wed Jun 01 21:31:08 2016 +0200
+++ b/src/Pure/General/rat.ML	Wed Jun 01 22:35:51 2016 +0200
@@ -89,7 +89,7 @@
 fun ceil (Rat (p, q)) =
   (case Integer.div_mod p q of
     (m, 0) => m
-  | (m, _) => m);
+  | (m, _) => m + 1);
 
 end;