code: func (bc *BlockChain) FastSyncCommitHead(hash common.Hash) error {
  	// Make sure that both the block as well at its state trie exists
  	block := bc.GetBlockByHash(hash)
  	if block == nil {
  		return fmt.Errorf("non existent block [%x…]", hash[:4])
  	}
  	if _, err := trie.NewSecure(block.Root(), bc.stateCache.TrieDB()); err != nil {
  		return err
  	}
  	// If all checks out, manually set the head block
  	bc.chainmu.Lock()
  	bc.currentBlock.Store(block)
  	bc.chainmu.Unlock()
  
  	log.Info("Committed new head block", "number", block.Number(), "hash", hash)
  	return nil
  }
docs: // FastSyncCommitHead sets the current head block to the one defined by the hash
  // irrelevant what the chain contents were prior.