Function next_number 

pub fn next_number(existing: &[Invoice], prefix: &str, year: i16) -> String

The number after the last one given with the same prefix: prefix (else the year, “2026-”) and three digits. With the usual prefix, the year, the count restarts each January (2026-001, then 2027-001); with your own prefix it never restarts, and goes on from the last one whatever the year (F-041, F-042…), as the guide says (Clients, “Invoices”).