code: func (db *Database) insertPreimage(hash common.Hash, preimage []byte) {
  	if _, ok := db.preimages[hash]; ok {
  		return
  	}
  	db.preimages[hash] = common.CopyBytes(preimage)
  	db.preimagesSize += common.StorageSize(common.HashLength + len(preimage))
  }
docs: // insertPreimage writes a new trie node pre-image to the memory database if it's
  // yet unknown. The method will make a copy of the slice.
  //
  // Note, this method assumes that the database's lock is held!