code: func (c *Cell) getNumberFormat() *parsedNumberFormat {
  	if c.parsedNumFmt == nil || c.parsedNumFmt.numFmt != c.NumFmt {
  		c.parsedNumFmt = parseFullNumberFormatString(c.NumFmt)
  	}
  	return c.parsedNumFmt
  }
docs: // getNumberFormat will update the parsedNumFmt struct if it has become out of date, since a cell's NumFmt string is a
  // public field that could be edited by clients.