diff -r f00da558b78e -r 511df47bcadc src/Pure/package.scala
--- a/src/Pure/package.scala	Mon Jul 04 13:43:10 2011 +0200
+++ b/src/Pure/package.scala	Mon Jul 04 16:27:11 2011 +0200
@@ -6,6 +6,8 @@
 
 package object isabelle
 {
+  /* errors */
+
   object ERROR
   {
     def apply(message: String): Throwable = new RuntimeException(message)
@@ -17,6 +19,11 @@
         case _ => None
       }
   }
+
   def error(message: String): Nothing = throw ERROR(message)
+
+  def cat_error(msg1: String, msg2: String): Nothing =
+    if (msg1 == "") error(msg1)
+    else error(msg1 + "\n" + msg2)
 }