code: func (bc *BlockChain) GetUnclesInChain(block *types.Block, length int) []*types.Header {
  	uncles := []*types.Header{}
  	for i := 0; block != nil && i < length; i++ {
  		uncles = append(uncles, block.Uncles()...)
  		block = bc.GetBlock(block.ParentHash(), block.NumberU64()-1)
  	}
  	return uncles
  }
docs: // GetUnclesInChain retrieves all the uncles from a given block backwards until
  // a specific distance is reached.