func VersionVar(p *versionValue, name string, value versionValue, usage string) {
*p = value
flag.Var(p, name, usage)
// "--version" will be treated as "--version=true"
flag.Lookup(name).NoOptDefVal = "true"
}
// QuantityFlag is a helper that makes a quantity flag (using standard flag package).
// Will panic if defaultValue is not a valid quantity.
func QuantityFlag(flagName, defaultValue, description string) *Quantity {
q := MustParse(defaultValue)
flag.Var(NewQuantityFlagValue(&q), flagName, description)
return &q
}