You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: middleware_header_comment.go
    path of file: ./repos/ko/vendor/github.com/aws/smithy-go/transport/http
    
    the code of the file until where you have to start completion: package http

import (
	"context"
	"fmt"
	"net/http"

	"github.com/aws/smithy-go/middleware"
)

// WithHeaderComment instruments a middleware stack to append an HTTP field
// comment to the given header as specified in RFC 9110
// (https://www.rfc-editor.org/rfc/rfc9110#name-comments).
//
// The header is case-insensitive. I
    
func getOrAddHeaderComment(s *middleware.Stack) (*headerCommentMiddleware, error) {
	id := (*headerCommentMiddleware)(nil).ID()
	m, ok := s.Build.Get(id)
	if !ok {
		m := &headerCommentMiddleware{values: http.Header{}}
		if err := s.Build.Add(m, middleware.After); err != nil {
			return nil, fmt.Errorf("add build: %v", err)
		}

		return m, nil
	}

	hc, ok := m.(*headerCommentMiddleware)
	if !ok {
		return nil, fmt.Errorf("existing middleware w/ id %s is not *headerCommentMiddleware", id)
	}

	return hc, nil
}