code: func (lc *LightChain) SetHead(head uint64) {
  	lc.chainmu.Lock()
  	defer lc.chainmu.Unlock()
  
  	lc.hc.SetHead(head, nil)
  	lc.loadLastState()
  }
docs: // SetHead rewinds the local chain to a new head. Everything above the new
  // head will be deleted and the new one set.