code: func (api *API) GetSnapshotAtHash(hash common.Hash) (*Snapshot, error) {
  	header := api.chain.GetHeaderByHash(hash)
  	if header == nil {
  		return nil, errUnknownBlock
  	}
  	return api.clique.snapshot(api.chain, header.Number.Uint64(), header.Hash(), nil)
  }
docs: // GetSnapshotAtHash retrieves the state snapshot at a given block.