def check_validity_of(rows)
        raise "rows need to be arrays"             if !rows_are_arrays?(rows)
        raise "needs at least one row"             if rows.empty?
        raise "rows must be of same column size. Use "+
              "'equalize: true' flag to fix this." if !same_column_size?(rows)
      end
def check_validity_of(rows)
        raise "rows need to be arrays"             if !rows_are_arrays?(rows)
        raise "needs at least one row"             if rows.empty?
        raise "rows must be of same column size. Use "+
              "'equalize: true' flag to fix this." if !same_column_size?(rows)
      end
Checks whether the rows are valid, that is
   * same size
   * not nil
   * at least one row