// hide versions, since the "list" and "latest" endpoints are not
		// authenticated.
		return
	} else if errors.As(err, &retractErr) {
		if len(retractErr.Rationale) == 0 {
			m.Retracted = []string{"retracted by module author"}
		} else {
			m.Retracted = retractErr.Rationale
		}
	} else if m.Error == nil {
		m.Error = &modinfo.ModuleError{Err: err.Error()}
	}
}

The -retracted flag causes list to report information about retracted
module versions. When -retracted is used with -f or -json, the Retracted
field will be set to a string explaining why the version was retracted.
The string is taken from comments on the retract directive in the
module's go.mod file. When -retracted is used with -versions, retracted
versions are listed together with unretracted versions. The -retracted
flag may be used with or without -m.

// CheckRetractions returns an error if module m has been retracted by
// its author.
func CheckRetractions(ctx context.Context, m module.Version) (err error) {
	defer func() {
		if retractErr := (*ModuleRetractedError)(nil); err == nil || errors.As(err, &retractErr) {
			return
		}
		// Attribute the error to the version being checked, not the version from

	if err != nil {
		isRetracted = func(string) bool { return false }
	}

	// tagToVersion returns the version obtained by trimming tagPrefix from tag.
	// If the tag is invalid, retracted, or a pseudo-version, tagToVersion returns
	// an empty version.
	tagToVersion := func(tag string) (v string, tagIsCanonical bool) {
		if !strings.HasPrefix(tag, tagPrefix) {
			return "", false
		}

		downgraded = append(downgraded, r)
	}

	// The downgrades we computed above only downgrade to versions enumerated by
	// reqs.Previous. However, reqs.Previous omits some versions — such as
	// pseudo-versions and retracted versions — that may be selected as transitive
	// requirements of other modules.
	//
	// If one of those requirements pulls the version back up above the version

	37:  "signal 37",
	38:  "signal 38",
	39:  "signal 39",
	48:  "signal 48",
	49:  "signal 49",
	58:  "signal 58",
	59:  "CPU Failure Predicted",
	60:  "monitor mode granted",
	61:  "monitor mode retracted",
	62:  "sound completed",
	63:  "secure attention",
	255: "signal 255",

//
// require directives are not de-duplicated. That's left up to higher-level
// logic (MVS).
//
// retract directives are not de-duplicated since comments are
// meaningful, and versions may be retracted multiple times.
func (f *File) removeDups() {
	removeDups(f.Syntax, &f.Exclude, &f.Replace)
}

func removeDups(syntax *FileSyntax, exclude *[]*Exclude, replace *[]*Replace) {
	kill := make(map[*Line]bool)