code: public int addMergeRegion (@Nonnegative final int nFirstRow,
                               @Nonnegative final int nLastRow,
                               @Nonnegative final int nFirstCol,
                               @Nonnegative final int nLastCol)
    {
      return m_aLastSheet.addMergedRegion (new CellRangeAddress (nFirstRow, nLastRow, nFirstCol, nLastCol));
    }
docs: Adds a merged region of cells (hence those cells form one)
  
  @param nFirstRow
  Index of first row
  @param nLastRow
  Index of last row (inclusive), must be equal to or larger than
  {@code nFirstRow}
  @param nFirstCol
  Index of first column
  @param nLastCol
  Index of last column (inclusive), must be equal to or larger than
  {@code nFirstCol}
  @return index of this region