code: func (f *FlagSet) IPMask(name string, value net.IPMask, usage string) *net.IPMask {
  	p := new(net.IPMask)
  	f.IPMaskVarP(p, name, "", value, usage)
  	return p
  }
docs: // IPMask defines an net.IPMask flag with specified name, default value, and usage string.
  // The return value is the address of an net.IPMask variable that stores the value of the flag.