t521.scala:10: error: class PlainFile needs to be abstract, since method path in class AbstractFile of type => String is not defined
class PlainFile(val file : File) extends AbstractFile {}
      ^
t521.scala:13: error: overriding value file in class PlainFile of type java.io.File;
 value file needs `override' modifier
final class ZipArchive(val file : File, archive : ZipFile) extends PlainFile(file) {
                           ^
t521.scala:13: error: class ZipArchive needs to be abstract, since method path in class AbstractFile of type => String is not defined
final class ZipArchive(val file : File, archive : ZipFile) extends PlainFile(file) {
            ^
t521.scala:15: error: overriding value path in class VirtualFile of type String;
 method path needs to be a stable, immutable value
    override def path = "";
                 ^
four errors found