code: private static File getFileForKey(Key k) {
      final int off = k._kb[0] == Key.CHK ? water.fvec.Vec.KEY_PREFIX_LEN : 0;
      assert new String(k._kb, off, KEY_PREFIX_LENGTH).equals(KEY_PREFIX) : "Not an NFS key: " + k;
      String s = new String(k._kb, KEY_PREFIX_LENGTH + off, k._kb.length - (KEY_PREFIX_LENGTH + off));
      return new File(s);
    }
docs: Returns the file for given key.