code: public void add(Key k, byte[] vByte) throws IllegalArgumentException {
      vByte = Arrays.copyOf(vByte, vByte.length);
  
      if (key == null) {
        key = new Key(k);
        add(k.getTimestamp(), vByte);
      } else if (key.equals(k, PartialKey.ROW_COLFAM_COLQUAL_COLVIS)) {
        add(k.getTimestamp(), vByte);
      } else {
        throw new IllegalArgumentException();
      }
    }
docs: When empty, the first key added sets the row+column. After this all keys added must have the
  same row+column.
  
  @param k Key to be added to buffer
  @param vByte Value to be added to buffer