diff -r a38c74b0886d -r f83c1842a559 src/Pure/Tools/update_comments.scala
--- a/src/Pure/Tools/update_comments.scala	Sun Jan 14 20:39:27 2018 +0100
+++ b/src/Pure/Tools/update_comments.scala	Sun Jan 14 20:58:41 2018 +0100
@@ -20,8 +20,8 @@
         case tok :: rest if tok.source == "--" || tok.source == Symbol.comment =>
           rest.dropWhile(_.is_space) match {
             case tok1 :: rest1 if tok1.is_text =>
-              val res = Symbol.comment + Symbol.space + Library.cartouche(tok1.content)
-              update(rest1, res :: result)
+              val txt = Symbol.trim_blanks(tok1.content)
+              update(rest1, (Symbol.comment + Symbol.space + Library.cartouche(txt)) :: result)
             case _ => update(rest, tok.source :: result)
           }
         case tok :: rest => update(rest, tok.source :: result)