diff --git a/src/Tools/Setup/isabelle/setup/Library.java b/src/Tools/Setup/isabelle/setup/Library.java
--- a/src/Tools/Setup/isabelle/setup/Library.java
+++ b/src/Tools/Setup/isabelle/setup/Library.java
@@ -1,52 +1,48 @@
 /*  Title:      Tools/Setup/isabelle/setup/Library.java
     Author:     Makarius
 
 Basic library.
 */
 
 package isabelle.setup;
 
 
 import java.util.Arrays;
 import java.util.LinkedList;
 import java.util.List;
 
 
 public class Library
 {
     public static String cat_lines(Iterable<? extends CharSequence> lines)
     {
         return String.join("\n", lines);
     }
 
     public static List<String> split_lines(String str)
     {
         if (str.isEmpty()) { return List.of(); }
         else {
             List<String> result = new LinkedList<String>();
             result.addAll(Arrays.asList(str.split("\\n")));
             return List.copyOf(result);
         }
     }
 
     public static String prefix_lines(String prfx, String str)
     {
         if (str.isEmpty()) { return str; }
         else {
-            StringBuilder result = new StringBuilder();
-            for (String s : split_lines(str)) {
-                result.append(prfx);
-                result.append(s);
-                result.append('\n');
-            }
-            return result.toString();
+            List<String> lines = new LinkedList<String>();
+            for (String line : split_lines(str)) { lines.add(prfx + line); }
+            return cat_lines(lines);
         }
     }
 
     public static String trim_line(String s)
     {
         if (s.endsWith("\r\n")) { return s.substring(0, s.length() - 2); }
         else if (s.endsWith("\r") || s.endsWith("\n")) { return s.substring(0, s.length() - 1); }
         else { return s; }
     }
 }
\ No newline at end of file