_, ok = patchMap[key]
		return !ok
	}
}

// RequireMetadataKeyUnchanged creates a precondition function that fails
// if the metadata.key is present in the patch (indicating its value
// has changed).
func RequireMetadataKeyUnchanged(key string) PreconditionFunc {
	return func(patch interface{}) bool {
		patchMap, ok := patch.(map[string]interface{})
		if !ok {