+-- Given a <= function, group finds maximal contiguous up-runs
+-- or down-runs in the input list.
+-- It's stable, in the sense that it never re-orders equal elements
+--
+-- Date: Mon, 12 Feb 1996 15:09:41 +0000
+-- From: Andy Gill <andy@dcs.gla.ac.uk>
+-- Here is a `better' definition of group.