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

import (
	"context"
	"fmt"
	"sort"
	"strings"
)

func validateExtensions(ctx context.Context, extensions map[string]interface{}) error { // FIXME: newtype + Validate(...)
	allowed := getValidationOptions(ctx).extraSiblingFieldsAllowed

	var unknowns []str
    
func validateExtensions(ctx context.Context, extensions map[string]interface{}) error { // FIXME: newtype + Validate(...)
	allowed := getValidationOptions(ctx).extraSiblingFieldsAllowed

	var unknowns []string
	for k := range extensions {
		if strings.HasPrefix(k, "x-") {
			continue
		}
		if allowed != nil {
			if _, ok := allowed[k]; ok {
				continue
			}
		}
		unknowns = append(unknowns, k)
	}

	if len(unknowns) != 0 {
		sort.Strings(unknowns)
		return fmt.Errorf("extra sibling fields: %+v", unknowns)
	}

	return nil
}