code: func (api *PrivateDebugAPI) Preimage(ctx context.Context, hash common.Hash) (hexutil.Bytes, error) {
  	if preimage := rawdb.ReadPreimage(api.eth.ChainDb(), hash); preimage != nil {
  		return preimage, nil
  	}
  	return nil, errors.New("unknown preimage")
  }
docs: // Preimage is a debug API function that returns the preimage for a sha3 hash, if known.