code: 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
docs: Checks whether the rows are valid, that is
     * same size
     * not nil
     * at least one row