diff --git a/tools/utils.scala b/tools/utils.scala
--- a/tools/utils.scala
+++ b/tools/utils.scala
@@ -1,31 +1,31 @@
 /* Author: Fabian Huch, TU Muenchen
 
 Utilities.
  */
 package afp
 
 
 import isabelle._
 
 import scala.collection.immutable.ListMap
 
 
 object Utils
 {
   def group_sorted[A, K](l: List[A], f: A => K): ListMap[K, List[A]] =
-    l.foldRight(ListMap.empty[K, List[A]]) {
-      case (a, m) =>
+    l.foldLeft(ListMap.empty[K, List[A]]) {
+      case (m, a) =>
         m.updatedWith(f(a)) {
-          case Some(as) => Some(a :: as)
+          case Some(as) => Some(as :+ a)
           case None => Some(List(a))
         }
     }
 
   def grouped_sorted[A, K](l: List[A], f: A => K): ListMap[K, A] =
     group_sorted(l, f).map {
       case (k, v :: Nil) => k -> v
       case (k, vs) => error("Not distinct for " + quote(k.toString) + ": " + commas_quote(vs.map(_.toString)))
     }
 
   def the_entry[K, V](m: Map[K, V], k: K): V = m.getOrElse(k, error("Expected key " + quote(k.toString)))
 }
\ No newline at end of file