code: func (fs *FlagSet) Uint64Var(p *uint64, names []string, value uint64, usage string) {
  	fs.Var(newUint64Value(value, p), names, usage)
  }
docs: // Uint64Var defines a uint64 flag with specified name, default value, and usage string.
  // The argument p points to a uint64 variable in which to store the value of the flag.