src/Pure/Thy/thy_header.scala

   val USES = "uses"
   val BEGIN = "begin"
 
   val lexicon = Scan.Lexicon("%", "(", ")", ";", BEGIN, HEADER, IMPORTS, THEORY, USES)
 
-  sealed case class Header(val name: String, val imports: List[String], val uses: List[String])
+  final case class Header(val name: String, val imports: List[String], val uses: List[String])
 }
 
 
 class Thy_Header(symbols: Symbol.Interpretation) extends Outer_Parse.Parser
 {