code: func WriteTxLookupEntries(db ethdb.Writer, block *types.Block) {
  	for _, tx := range block.Transactions() {
  		if err := db.Put(txLookupKey(tx.Hash()), block.Number().Bytes()); err != nil {
  			log.Crit("Failed to store transaction lookup entry", "err", err)
  		}
  	}
  }
docs: // WriteTxLookupEntries stores a positional metadata for every transaction from
  // a block, enabling hash based transaction and receipt lookups.