diff -r d515b6140381 -r b34be3010273 src/Tools/Graphview/layout.scala
--- a/src/Tools/Graphview/layout.scala	Sun Feb 11 15:03:25 2018 +0100
+++ b/src/Tools/Graphview/layout.scala	Sun Feb 11 18:09:17 2018 +0100
@@ -7,7 +7,7 @@
   Georg Sander, "Graph Layout through the VCG Tool", in: Graph Drawing,
   DIMACS International Workshop (GD'94), Springer LNCS 894, 1995.
 
-  http://dx.doi.org/10.1007/3-540-58950-3_371
+  https://doi.org/10.1007/3-540-58950-3_371
   ftp://ftp.cs.uni-sb.de/pub/graphics/vcg/doc/tr-A03-94.ps.gz
 */