diff -r e6997512ef6c -r e1d01b351724 src/Tools/Haskell/Markup.hs
--- a/src/Tools/Haskell/Markup.hs	Sat Nov 10 17:07:17 2018 +0100
+++ b/src/Tools/Haskell/Markup.hs	Sat Nov 10 17:12:09 2018 +0100
@@ -5,6 +5,8 @@
     LICENSE:    BSD 3-clause (Isabelle)
 
 Quasi-abstract markup elements.
+
+See also "$ISABELLE_HOME/src/Pure/PIDE/markup.ML".
 -}
 
 module Isabelle.Markup (