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: parser.go
    path of file: ./repos/go-git/internal/revision
    
    the code of the file until where you have to start completion: // Package revision extracts git revision from string
// More information about revision : https://www.kernel.org/pub/software/scm/git/docs/gitrevisions.html
package revision

import (
	"bytes"
	"fmt"
	"io"
	"regexp"
	"strconv"
	"time"
)

// ErrInvalidRevision is emitted if string doesn't match valid revision
type ErrInvalidRevision struct {
	s string
}

func (e *ErrInvalidRevision) Error() string {
	return "Revision invalid : " + e.s
}

// Revisioner represents a revision component.
// A revision is made of multiple revision components
// obtained after parsing a revision string,
// for instance revision "master~" will be converted in
// two revision components Ref and TildePath
type Revisioner interface {
}

// Ref represents a reference name : HEAD, master, <hash>
type Ref string

// TildePath represents ~, ~{n}
type TildePath struct {
	Depth int
}

// CaretPath represents ^, ^{n}
type Car
    
func (p *Parser) parseAt() (Revisioner, error) {
	var tok, nextTok token
	var lit, nextLit string
	var err error

	tok, _, err = p.scan()

	if err != nil {
		return nil, err
	}

	if tok != obrace {
		p.unscan()

		return Ref("HEAD"), nil
	}

	tok, lit, err = p.scan()

	if err != nil {
		return nil, err
	}

	nextTok, nextLit, err = p.scan()

	if err != nil {
		return nil, err
	}

	switch {
	case tok == word && (lit == "u" || lit == "upstream") && nextTok == cbrace:
		return AtUpstream{}, nil
	case tok == word && lit == "push" && nextTok == cbrace:
		return AtPush{}, nil
	case tok == number && nextTok == cbrace:
		n, _ := strconv.Atoi(lit)

		return AtReflog{n}, nil
	case tok == minus && nextTok == number:
		n, _ := strconv.Atoi(nextLit)

		t, _, err := p.scan()

		if err != nil {
			return nil, err
		}

		if t != cbrace {
			return nil, &ErrInvalidRevision{s: `missing "}" in @{-n} structure`}
		}

		return AtCheckout{n}, nil
	default:
		p.unscan()

		date := lit

		for {
			tok, lit, err = p.scan()

			if err != nil {
				return nil, err
			}

			switch {
			case tok == cbrace:
				t, err := time.Parse("2006-01-02T15:04:05Z", date)

				if err != nil {
					return nil, &ErrInvalidRevision{fmt.Sprintf(`wrong date "%s" must fit ISO-8601 format : 2006-01-02T15:04:05Z`, date)}
				}

				return AtDate{t}, nil
			case tok == eof:
				return nil, &ErrInvalidRevision{s: `missing "}" in @{<data>} structure`}
			default:
				date += lit
			}
		}