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: refs.go
    path of file: ./repos/kin-openapi/openapi3
    
    the code of the file until where you have to start completion: package openapi3

import (
	"context"
	"encoding/json"
	"fmt"
	"sort"

	"github.com/go-openapi/jsonpointer"
	"github.com/perimeterx/marshmallow"
)

// CallbackRef represents either a Callback or a $ref to a Callback.
// When serializing and both fields are set, Ref is preferred over Value.
type CallbackRef struct {
	Ref   string
	Value *Callback
	extra []string
    
func (x *ParameterRef) Validate(ctx context.Context, opts ...ValidationOption) error {
	ctx = WithValidationOptions(ctx, opts...)
	if extra := x.extra; len(extra) != 0 {
		extras := make([]string, 0, len(extra))
		allowed := getValidationOptions(ctx).extraSiblingFieldsAllowed
		for _, ex := range extra {
			if allowed != nil {
				if _, ok := allowed[ex]; ok {
					continue
				}
			}
			extras = append(extras, ex)
		}
		if len(extras) != 0 {
			return fmt.Errorf("extra sibling fields: %+v", extras)
		}
	}
	if v := x.Value; v != nil {
		return v.Validate(ctx)
	}
	return foundUnresolvedRef(x.Ref)
}