定义如下，根据是否有交易，可以分为tick和transaction两类entry.
pub struct Entry {
/// The number of hashes since the previous Entry ID.
pub num_hashes: u64,//做了多少次hash
/// The SHA-256 hash `num_hashes` after the previous Entry ID.
pub hash: Hash,//此次entry的hash大小
/// An unordered list of transactions that were observed before the Entry ID was
/// generated. They may have been observed before a previous Entry ID but were
/// pushed back into this list to ensure deterministic interpretation of the ledger.
pub transactions: Vec<VersionedTransaction>,//交易组成的数组
}
