r"""
        Combine columns belonging to a group to a single multicolumn entry
        according to self.multicolumn_format

        e.g.:
        a &  &  & b & c &
        will become
        \multicolumn{3}{l}{a} & b & \multicolumn{2}{l}{c}

def _format_multicolumn(self, row, ilevels):
        r"""
        Combine columns belonging to a group to a single multicolumn entry
        according to self.multicolumn_format

        e.g.:
        a &  &  & b & c &
        will become
        \multicolumn{3}{l}{a} & b & \multicolumn{2}{l}{c}
        """
        row2 = list(row[:ilevels])
        ncol = 1
        coltext = ''

        def append_col():
            # write multicolumn if needed
            if ncol > 1:
                row2.append('\\multicolumn{{{ncol:d}}}{{{fmt:s}}}{{{txt:s}}}'
                            .format(ncol=ncol, fmt=self.multicolumn_format,
                                    txt=coltext.strip()))
            # don't modify where not needed
            else:
                row2.append(coltext)
        for c in row[ilevels:]:
            # if next col has text, write the previous
            if c.strip():
                if coltext:
                    append_col()
                coltext = c
                ncol = 1
            # if not, add it to the previous multicolumn
            else:
                ncol += 1
        # write last column name
        if coltext:
            append_col()
        return row2