code: func (s *PullRequestsService) CreateCommentInReplyTo(ctx context.Context, owner string, repo string, number int, body string, commentID int64) (*PullRequestComment, *Response, error) {
  	comment := &struct {
  		Body      string `json:"body,omitempty"`
  		InReplyTo int64  `json:"in_reply_to,omitempty"`
  	}{
  		Body:      body,
  		InReplyTo: commentID,
  	}
  	u := fmt.Sprintf("repos/%v/%v/pulls/%d/comments", owner, repo, number)
  	req, err := s.client.NewRequest("POST", u, comment)
  	if err != nil {
  		return nil, nil, err
  	}
  
  	c := new(PullRequestComment)
  	resp, err := s.client.Do(ctx, req, c)
  	if err != nil {
  		return nil, resp, err
  	}
  
  	return c, resp, nil
  }
docs: // CreateCommentInReplyTo creates a new comment as a reply to an existing pull request comment.
  //
  // GitHub API docs: https://developer.github.com/v3/pulls/comments/#alternative-input