code: private void checkContract( Path path ) {
          if( !path.isAbsolute() ) {
              throw new IllegalArgumentException( "EightFileSystem only works with absolute paths " + path );
          }
  
          if( ( (EightyFileSystem) path.getFileSystem() ).get80() != this ) {
              throw new IllegalArgumentException( "path called on incorrect 80" );
          }
      }
docs: /*
  -------------------------------------------------