Write specified namedtuples into an Excel spreadsheet.

def prt_mdtbl(self, nts, prt_flds=None, prt=sys.stdout):
        """Write specified namedtuples into an Excel spreadsheet."""
        if prt_flds is None:
            prt_flds = self.prt_flds
        prtfmt = "|{DATA}|\n".format(DATA="|".join(self.fld2prtfmt[f] for f in prt_flds))
        prt_txt(prt, nts, prtfmt=prtfmt)