code: func (q InternalQuad) Set(d quad.Direction, v graph.Value) {
  	switch d {
  	case quad.Subject:
  		q.Subject = v
  	case quad.Predicate:
  		q.Predicate = v
  	case quad.Object:
  		q.Object = v
  	case quad.Label:
  		q.Label = v
  	default:
  		panic(d)
  	}
  }
docs: // Set assigns a specified direction of the quad to a given value.