diff --git a/src/mocks/TimelineMock.ts b/src/mocks/TimelineMock.ts
index 444a55cf..d38e37cc 100644
--- a/src/mocks/TimelineMock.ts
+++ b/src/mocks/TimelineMock.ts
@@ -7,6 +7,7 @@ export function eventId(i: number): string {
     return `$event${i}`;
 }
 
+/** `from` is included, `to` is excluded */
 export function eventIds(from: number, to: number): string[] {
     return [...Array(to-from).keys()].map(i => eventId(i + from));
 }