code: private boolean isVersionEnabled(File versionDir) throws IllegalArgumentException {
          if (!versionDir.exists()) {
              throw new IllegalArgumentException("The versionDir " + versionDir.getName() + " does not exist.");
          }
          File[] relevantFile = versionDir.listFiles(new FileFilter() {
              public boolean accept(File pathName) {
                  return pathName.getName().equals(DISABLED_MARKER_NAME);
              }
          });
          return relevantFile.length == 0;
      }
docs: Inspects the specified versionDir to see if it has been marked as disabled
  (via a .disabled file in the directory). If the file is absent, the store is
  assumed to be enabled.
  
  @param versionDir to inspect
  @return true if the specified version is enabled, false otherwise
  @throws IllegalArgumentException if the version does not exist