code: function(formatCode) {
      // check if default format
      var index = NumFmtXform.getDefaultFmtId(formatCode);
      if (index !== undefined) return index;
  
      // check if already in
      index = this.index.numFmt[formatCode];
      if (index !== undefined) return index;
  
  
      index = this.index.numFmt[formatCode] = NUMFMT_BASE + this.model.numFmts.length;
      var xml = this.map.numFmt.toXml({id: index, formatCode: formatCode});
      this.model.numFmts.push(xml);
      return index;
    }
docs: ========================================================================= Number Formats