/**
 * Per-chat outbound serialization. When two concurrent reply paths target the
 * same chatId (e.g., user sends two messages in rapid succession), this queue
 * ensures the second `enqueue(chatId, fn)` waits until the first has settled
 * before its `fn` runs — chunks of the two replies don't interleave.
 *
 * Different chatIds run in parallel: enqueue(A, fnA) and enqueue(B, fnB) start
 * concurrently regardless of submission order.
 *
 * Chains survive errors: if fn throws or its returned promise rejects, the next
 * enqueue for the same chatId still runs.
 */
export declare class PerChatSendQueue {
    private readonly tails;
    enqueue<T>(chatId: string, fn: () => Promise<T>): Promise<T>;
    size(): number;
}
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