diff -r 7cdd4d59e95c -r fb3f13a9c756 src/Pure/Isar/parse.scala
--- a/src/Pure/Isar/parse.scala	Tue Oct 24 21:20:55 2017 +0200
+++ b/src/Pure/Isar/parse.scala	Wed Oct 25 11:35:48 2017 +0200
@@ -72,8 +72,8 @@
     def path: Parser[String] =
       atom("file name/path specification", tok => tok.is_embedded && Path.is_wellformed(tok.content))
 
-    def theory_name: Parser[String] =
-      atom("theory name", tok => tok.is_name && Path.is_wellformed(tok.content))
+    def session_name: Parser[String] = atom("session name", _.is_system_name)
+    def theory_name: Parser[String] = atom("theory name", _.is_system_name)
 
     private def tag_name: Parser[String] =
       atom("tag name", tok =>