src/Tools/Graphview/shapes.scala

     new BasicStroke(1, BasicStroke.CAP_BUTT, BasicStroke.JOIN_ROUND)
 
   def shape(info: Layout.Info): Rectangle2D.Double =
     new Rectangle2D.Double(info.x - info.width2, info.y - info.height2, info.width, info.height)
 
-  def highlight_node(gfx: Graphics2D, graphview: Graphview, node: Graph_Display.Node)
+  def highlight_node(gfx: Graphics2D, graphview: Graphview, node: Graph_Display.Node): Unit =
   {
     val metrics = graphview.metrics
     val extra = metrics.char_width
     val info = graphview.layout.get_node(node)
 

       info.y - info.height2 - extra,
       info.width + 2 * extra,
       info.height + 2 * extra))
   }
 
-  def paint_node(gfx: Graphics2D, graphview: Graphview, node: Graph_Display.Node)
+  def paint_node(gfx: Graphics2D, graphview: Graphview, node: Graph_Display.Node): Unit =
   {
     val metrics = graphview.metrics
     val info = graphview.layout.get_node(node)
     val c = graphview.node_color(node)
     val s = shape(info)

       gfx.drawString(Word.bidi_override(line), x, y)
       y += metrics.height.ceil.toInt
     }
   }
 
-  def paint_dummy(gfx: Graphics2D, graphview: Graphview, info: Layout.Info)
+  def paint_dummy(gfx: Graphics2D, graphview: Graphview, info: Layout.Info): Unit =
   {
     gfx.setStroke(default_stroke)
     gfx.setColor(graphview.dummy_color)
     gfx.draw(shape(info))
   }
 
   object Straight_Edge
   {
-    def paint(gfx: Graphics2D, graphview: Graphview, edge: Graph_Display.Edge)
+    def paint(gfx: Graphics2D, graphview: Graphview, edge: Graph_Display.Edge): Unit =
     {
       val p = graphview.layout.get_node(edge._1)
       val q = graphview.layout.get_node(edge._2)
       val ds =
       {

 
   object Cardinal_Spline_Edge
   {
     private val slack = 0.1
 
-    def paint(gfx: Graphics2D, graphview: Graphview, edge: Graph_Display.Edge)
+    def paint(gfx: Graphics2D, graphview: Graphview, edge: Graph_Display.Edge): Unit =
     {
       val p = graphview.layout.get_node(edge._1)
       val q = graphview.layout.get_node(edge._2)
       val ds =
       {

         case None => None
         case Some(line) => binary_search(line, shape)
       }
     }
 
-    def paint(gfx: Graphics2D, path: GeneralPath, shape: Shape)
+    def paint(gfx: Graphics2D, path: GeneralPath, shape: Shape): Unit =
     {
       position(path, shape) match {
         case None =>
         case Some(at) =>
           val arrow = new GeneralPath(Path2D.WIND_EVEN_ODD, 3)