code: func (f *FlagSet) usage() {
  	if f.Usage == nil {
  		if f == CommandLine {
  			Usage()
  		} else {
  			defaultUsage(f)
  		}
  	} else {
  		f.Usage()
  	}
  }
docs: // usage calls the Usage method for the flag set if one is specified,
  // or the appropriate default usage function otherwise.