code: func (p Primitive) GetDirection(d quad.Direction) uint64 {
  	switch d {
  	case quad.Subject:
  		return p.Subject
  	case quad.Predicate:
  		return p.Predicate
  	case quad.Object:
  		return p.Object
  	case quad.Label:
  		return p.Label
  	}
  	panic("unknown direction")
  }
docs: //go:generate protoc --proto_path=$GOPATH/src:. --gogo_out=. primitive.proto