code: func (api *API) GetSignersAtHash(hash common.Hash) ([]common.Address, error) {
  	header := api.chain.GetHeaderByHash(hash)
  	if header == nil {
  		return nil, errUnknownBlock
  	}
  	snap, err := api.clique.snapshot(api.chain, header.Number.Uint64(), header.Hash(), nil)
  	if err != nil {
  		return nil, err
  	}
  	return snap.signers(), nil
  }
docs: // GetSignersAtHash retrieves the list of authorized signers at the specified block.