diff --git a/src/Tools/Graphview/graph_file.scala b/src/Tools/Graphview/graph_file.scala
--- a/src/Tools/Graphview/graph_file.scala
+++ b/src/Tools/Graphview/graph_file.scala
@@ -1,49 +1,50 @@
 /*  Title:      Tools/Graphview/graph_file.scala
     Author:     Makarius
 
 File system operations for graph output.
 */
 
 package isabelle.graphview
 
 
 import isabelle._
 
 import java.io.{File => JFile}
 import java.awt.{Color, Graphics2D}
 
 
 object Graph_File {
   def write(file: JFile, graphview: Graphview): Unit = {
     val box = graphview.bounding_box()
     val w = box.width.ceil.toInt
     val h = box.height.ceil.toInt
 
     def paint(gfx: Graphics2D): Unit = {
       gfx.setColor(graphview.background_color)
       gfx.fillRect(0, 0, w, h)
       gfx.translate(- box.x, - box.y)
       graphview.paint(gfx)
     }
 
     val name = file.getName
     if (File.is_png(name)) Graphics_File.write_png(file, paint, w, h)
     else if (File.is_pdf(name)) Graphics_File.write_pdf(file, paint, w, h)
     else error("Bad type of file: " + quote(name) + " (.png or .pdf expected)")
   }
 
   def write(options: Options, file: JFile, graph: Graph_Display.Graph): Unit = {
-    val the_options = options
+    val graphview_options = options
     val graphview =
-      new Graphview(graph.transitive_reduction_acyclic) { def options = the_options }
+      new Graphview(graph.transitive_reduction_acyclic) {
+        def options: Options = graphview_options
+      }
     graphview.update_layout()
-
     write(file, graphview)
   }
 
   def make_pdf(options: Options, graph: Graph_Display.Graph): Bytes =
     Isabelle_System.with_tmp_file("graph", ext = "pdf") { graph_file =>
       write(options, graph_file.file, graph)
       Bytes.read(graph_file)
     }
 }