code: func WriteCanonicalHash(db ethdb.Writer, hash common.Hash, number uint64) {
  	if err := db.Put(headerHashKey(number), hash.Bytes()); err != nil {
  		log.Crit("Failed to store number to hash mapping", "err", err)
  	}
  }
docs: // WriteCanonicalHash stores the hash assigned to a canonical block number.