code: func GetBlock(rl Reader, index uint64) *cb.Block {
  	iterator, _ := rl.Iterator(&ab.SeekPosition{
  		Type: &ab.SeekPosition_Specified{
  			Specified: &ab.SeekSpecified{Number: index},
  		},
  	})
  	if iterator == nil {
  		return nil
  	}
  	defer iterator.Close()
  	block, status := iterator.Next()
  	if status != cb.Status_SUCCESS {
  		return nil
  	}
  	return block
  }
docs: // GetBlock is a utility method for retrieving a single block