src/Pure/Admin/build_cygwin.scala

   val packages: List[String] =
     List("curl", "libgmp-devel", "nano", "perl", "perl-libwww-perl", "rlwrap", "unzip")
 
   def build_cygwin(progress: Progress,
     mirror: String = default_mirror,
-    more_packages: List[String] = Nil)
+    more_packages: List[String] = Nil): Unit =
   {
     require(Platform.is_windows, "Windows platform expected")
 
     Isabelle_System.with_tmp_dir("cygwin")(tmp_dir =>
       {