src/Pure/Isar/token.scala

--- a/src/Pure/Isar/token.scala	Tue Oct 24 21:20:55 2017 +0200
+++ b/src/Pure/Isar/token.scala	Wed Oct 25 11:35:48 2017 +0200
@@ -305,4 +305,6 @@
     else if (kind == Token.Kind.CARTOUCHE) Scan.Parsers.cartouche_content(source)
     else if (kind == Token.Kind.COMMENT) Scan.Parsers.comment_content(source)
     else source
+
+  def is_system_name: Boolean = is_name && Path.is_wellformed(content)
 }