diff -r 63c9c6ceb7a3 -r 3b2daa7bf9f4 src/Pure/Isar/document_structure.scala
--- a/src/Pure/Isar/document_structure.scala	Wed Aug 29 18:53:29 2018 +0200
+++ b/src/Pure/Isar/document_structure.scala	Wed Aug 29 18:53:58 2018 +0200
@@ -21,7 +21,8 @@
   case class Atom(length: Int) extends Document
 
   private def is_theory_command(keywords: Keyword.Keywords, command: Command): Boolean =
-    command.span.is_kind(keywords, kind => Keyword.theory(kind) && !Keyword.theory_end(kind))
+    command.span.is_kind(keywords,
+      kind => Keyword.theory(kind) && !Keyword.theory_end(kind), false)