code: func (fs *FlagSet) Require(nArgRequirementType nArgRequirementType, nArg int) {
  	fs.nArgRequirements = append(fs.nArgRequirements, nArgRequirement{nArgRequirementType, nArg})
  }
docs: // Require adds a requirement about the number of arguments for the FlagSet.
  // The first parameter can be Exact, Max, or Min to respectively specify the exact,
  // the maximum, or the minimal number of arguments required.
  // The actual check is done in FlagSet.CheckArgs().