code: private static File getFileForKey(Key k) {
      final int len = KEY_PREFIX_LENGTH+1; // Strip key prefix & leading slash
      final int off = k._kb[0]==Key.DVEC ? water.fvec.Vec.KEY_PREFIX_LEN : 0;
      String s = new String(k._kb,len+off,k._kb.length-(len+off));
      return new File(s);
    }
docs: Returns the file for given key.