BookStateFor: StorageDescriptor<[Key: Enum<{
    Here: undefined;
    Parent: undefined;
    Sibling: number;
    Snowbridge: FixedSizeBinary<32>;
}>], {
    begin: number;
    count: number;
    end: number;
    message_count: bigint;
    ready_neighbours?: {
        next: Enum<{
            Here: undefined;
            Parent: undefined;
            Sibling: number;
            Snowbridge: FixedSizeBinary<32>;
        }>;
        prev: Enum<{
            Here: undefined;
            Parent: undefined;
            Sibling: number;
            Snowbridge: FixedSizeBinary<32>;
        }>;
    };
    size: bigint;
}, false, never>

The index of the first and last (non-empty) pages.