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: context.go
    path of file: ./repos/corteza/server/vendor/github.com/gorilla/csrf
    
    the code of the file until where you have to start completion: // +build go1.7

package csrf

import (
	"context"
	"net/http"

	"gi
    
func contextGet(r *http.Request, key string) (interface{}, error) {
	val := r.Context().Value(key)
	if val == nil {
		return nil, errors.Errorf("no value exists in the context for key %q", key)
	}

	return val, nil
}