/**
 * Fancy ID generator that creates 20-character string identifiers with the
 * following properties:
 *
 * 1. They're based on timestamp so that they sort *after* any existing ids.
 * 2. They contain 72-bits of random data after the timestamp so that IDs won't
 *    collide with other clients' IDs.
 * 3. They sort *lexicographically* (so the timestamp is converted to characters
 *    that will sort properly).
 * 4. They're monotonically increasing. Even if you generate more than one in
 *    the same timestamp, the latter ones will sort after the former ones. We do
 *    this by using the previous random bits but "incrementing" them by 1 (only
 *    in the case of a timestamp collision).
 */
export declare const nextPushId: (now: number) => string;