diff --git a/src/prelude/array.ts b/src/prelude/array.ts
index 8536e486d..69b52fde8 100644
--- a/src/prelude/array.ts
+++ b/src/prelude/array.ts
@@ -18,8 +18,12 @@ export function erase<T>(x: T, xs: T[]): T[] {
 	return xs.filter(y => x !== y);
 }
 
-export function setDifference<T>(xs: T[], ys: T[]): T[] {
-	return xs.filter(x => !ys.includes(x));
+/**
+ * Finds the array of all elements in the first array not contained in the second array.
+ * The order of result values are determined by the first array.
+ */
+export function difference<T>(includes: T[], excludes: T[]): T[] {
+	return includes.filter(x => !excludes.includes(x));
 }
 
 export function unique<T>(xs: T[]): T[] {
diff --git a/src/remote/activitypub/models/note.ts b/src/remote/activitypub/models/note.ts
index ce9a4daf1..7501bf1a8 100644
--- a/src/remote/activitypub/models/note.ts
+++ b/src/remote/activitypub/models/note.ts
@@ -13,7 +13,7 @@ import htmlToMFM from '../../../mfm/html-to-mfm';
 import Emoji from '../../../models/emoji';
 import { ITag } from './tag';
 import { toUnicode } from 'punycode';
-import { unique, concat, setDifference } from '../../../prelude/array';
+import { unique, concat, difference } from '../../../prelude/array';
 
 const log = debug('misskey:activitypub');
 
@@ -181,7 +181,7 @@ async function extractEmojis(tags: ITag[], host_: string) {
 
 async function extractMentionedUsers(actor: IRemoteUser, to: string[], cc: string[], resolver: Resolver) {
 	const ignoreUris = ['https://www.w3.org/ns/activitystreams#Public', `${actor.uri}/followers`];
-	const uris = setDifference(unique(concat([to || [], cc || []])), ignoreUris);
+	const uris = difference(unique(concat([to || [], cc || []])), ignoreUris);
 
 	const users = await Promise.all(
 		uris.map(async uri => await resolvePerson(uri, null, resolver).catch(() => null))