code: public int addMergeRegionInCurrentRow (@Nonnegative final int nFirstCol, @Nonnegative final int nLastCol)
    {
      final int nCurrentRowIndex = getRowIndex ();
      return addMergeRegion (nCurrentRowIndex, nCurrentRowIndex, nFirstCol, nLastCol);
    }
docs: Add a merge region in the current row. Note: only the content of the first
  cell is used as the content of the merged cell!
  
  @param nFirstCol
  First column to be merged (inclusive). 0-based
  @param nLastCol
  Last column to be merged (inclusive). 0-based, must be larger than
  {@code nFirstCol}
  @return index of this region