/**
 * Checks that a payload contains at most one `plan` block. Slack rejects
 * multi-plan payloads with `invalid_blocks` / "Cannot have more than 1 plan
 * block" (surfaced on e.g. `chat.update`).
 * @param blocks - array of Block Kit blocks
 * @returns array of error messages (empty when zero or one `plan` block present)
 */
export declare function checkSinglePlanBlock(blocks: readonly {
    type?: string;
}[]): string[];
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