code: func (d *Dataset) Broadcast(name string, shardCount int) *Dataset {
  	if shardCount == 1 && len(d.Shards) == shardCount {
  		return d
  	}
  	ret := d.Flow.NewNextDataset(shardCount)
  	step := d.Flow.AddOneToAllStep(d, ret)
  	step.SetInstruction(name, instruction.NewBroadcast())
  	return ret
  }
docs: // Broadcast replicates itself to all shards.