code: func (f *Fpdf) AliasNbPages(aliasStr string) {
  	if aliasStr == "" {
  		aliasStr = "{nb}"
  	}
  	f.aliasNbPagesStr = aliasStr
  }
docs: // AliasNbPages defines an alias for the total number of pages. It will be
  // substituted as the document is closed. An empty string is replaced with the
  // string "{nb}".
  //
  // See the example for AddPage() for a demonstration of this method.