afford higher execution priority by default: defer proofs and thus stretch parallelism over whole document;

--- a/etc/options	Mon Jul 29 15:59:47 2013 +0200
+++ b/etc/options	Mon Jul 29 16:01:05 2013 +0200
@@ -123,7 +123,7 @@
 public option editor_chart_delay : real = 3.0
   -- "delay for chart repainting"
 
-public option editor_execution_priority : int = -2
+option editor_execution_priority : int = -1
   -- "execution priority of main document structure (e.g. 0, -1, -2)"
 
 option editor_execution_range : string = "visible"