diff --git a/src/tags.test.ts b/src/tags.test.ts
new file mode 100644
index 0000000..9c7e938
--- /dev/null
+++ b/src/tags.test.ts
@@ -0,0 +1,25 @@
+import { assertEquals } from '@/deps-test.ts';
+
+import { deleteTag, getTagSet, setTag } from './tags.ts';
+
+Deno.test('getTagSet', () => {
+  assertEquals(getTagSet([], 'p'), new Set());
+  assertEquals(getTagSet([['p', '123']], 'p'), new Set(['123']));
+  assertEquals(getTagSet([['p', '123'], ['p', '456']], 'p'), new Set(['123', '456']));
+  assertEquals(getTagSet([['p', '123'], ['p', '456'], ['q', '789']], 'p'), new Set(['123', '456']));
+});
+
+Deno.test('setTag', () => {
+  assertEquals(setTag([], ['p', '123']), [['p', '123']]);
+  assertEquals(setTag([['p', '123']], ['p', '123']), [['p', '123']]);
+  assertEquals(setTag([['p', '123'], ['p', '456']], ['p', '123']), [['p', '123'], ['p', '456']]);
+  assertEquals(setTag([['p', '123'], ['p', '456']], ['p', '789']), [['p', '123'], ['p', '456'], ['p', '789']]);
+});
+
+Deno.test('deleteTag', () => {
+  assertEquals(deleteTag([], ['p', '123']), []);
+  assertEquals(deleteTag([['p', '123']], ['p', '123']), []);
+  assertEquals(deleteTag([['p', '123']], ['p', '456']), [['p', '123']]);
+  assertEquals(deleteTag([['p', '123'], ['p', '123']], ['p', '123']), []);
+  assertEquals(deleteTag([['p', '123'], ['p', '456']], ['p', '456']), [['p', '123']]);
+});
diff --git a/src/tags.ts b/src/tags.ts
index 98efc7d..a55560c 100644
--- a/src/tags.ts
+++ b/src/tags.ts
@@ -11,4 +11,19 @@ function getTagSet(tags: string[][], tagName: string): Set<string> {
   return set;
 }
 
-export { getTagSet };
+/** Delete all occurences of the tag by its name/value pair. */
+function deleteTag(tags: readonly string[][], tag: string[]): string[][] {
+  return tags.filter(([name, value]) => !(name === tag[0] && value === tag[1]));
+}
+
+/** Add a tag to the list, replacing the name/value pair if it already exists. */
+function setTag(tags: readonly string[][], tag: string[]): string[][] {
+  const tagIndex = tags.findIndex(([name, value]) => name === tag[0] && value === tag[1]);
+  if (tagIndex === -1) {
+    return [...tags, tag];
+  } else {
+    return [...tags.slice(0, tagIndex), tag, ...tags.slice(tagIndex + 1)];
+  }
+}
+
+export { deleteTag, getTagSet, setTag };