Constant PAID_KEYS_KEPT 

pub const PAID_KEYS_KEPT: usize = _; // 1_024usize

How many of the keys it paid last a holder remembers, so as to refuse paying one of them again: the notes of four requests of the most notes a request may ask for, or of hundreds of usual ones, for about 67 KiB of the saved store. A key paid by a transfer still outgoing, or by a payment the holder still keeps, is refused however long ago it was paid.