Mirabelle now gives usage info when no arguments given

--- a/src/HOL/Mirabelle/lib/Tools/mirabelle	Sat Apr 14 23:52:17 2012 +0100
+++ b/src/HOL/Mirabelle/lib/Tools/mirabelle	Sat Apr 14 23:52:17 2012 +0100
@@ -67,6 +67,8 @@
 
 # options
 
+[ $# -eq 0 ] && usage
+
 while getopts "L:T:O:t:q?" OPT
 do
   case "$OPT" in