src/Pure/Thy/sessions.scala

   }
 
   def parse_root(path: Path): List[Entry] = Parsers.parse_root(path)
 
   def parse_root_entries(path: Path): List[Session_Entry] =
-    for (entry <- Parsers.parse_root(path) if entry.isInstanceOf[Session_Entry])
-    yield entry.asInstanceOf[Session_Entry]
+    Parsers.parse_root(path).flatMap(Library.as_subclass(classOf[Session_Entry]))
 
   def parse_roots(roots: Path): List[String] = {
     for {
       line <- split_lines(File.read(roots))
       if !(line == "" || line.startsWith("#"))