src/Pure/System/isabelle_system.ML

--- a/src/Pure/System/isabelle_system.ML	Sun Nov 28 16:15:31 2010 +0100
+++ b/src/Pure/System/isabelle_system.ML	Sun Nov 28 16:35:56 2010 +0100
@@ -41,7 +41,8 @@
 
 fun mkdirs path = system_command ("mkdir -p " ^ File.shell_path path);
 
-val mkdir = OS.FileSys.mkDir o File.platform_path;
+fun mkdir path =
+  if File.is_dir path then () else OS.FileSys.mkDir (File.platform_path path);
 
 fun copy_dir src dst =
   if File.eq (src, dst) then ()