src/Pure/Isar/token.scala

 
   def is_unfinished: Boolean = is_error &&
    (source.startsWith("\"") ||
     source.startsWith("`") ||
     source.startsWith("{*") ||
-    source.startsWith("(*"))
+    source.startsWith("(*") ||
+    source.startsWith(Symbol.open) ||
+    source.startsWith(Symbol.open_decoded))
 
   def is_begin: Boolean = is_keyword && source == "begin"
   def is_end: Boolean = is_keyword && source == "end"
 
   def content: String =