code: func Sub(r1, r2 ResourceTuple) (res ResourceTuple) {
  	res.Cores = r1.Cores - r2.Cores
  	res.Memory = r1.Memory - r2.Memory
  	res.Disk = r1.Disk - r2.Disk
  	return
  }
docs: // Sub returns a ResourceTuple representing the difference between two
  // ResourceTuples