code: func NormalizeEnvironment(env *vpython.Environment) error {
  	if env.Spec == nil {
  		env.Spec = &vpython.Spec{}
  	}
  	if err := NormalizeSpec(env.Spec, env.Pep425Tag); err != nil {
  		return err
  	}
  
  	if env.Runtime == nil {
  		env.Runtime = &vpython.Runtime{}
  	}
  
  	sort.Sort(pep425TagSlice(env.Pep425Tag))
  	return nil
  }
docs: // NormalizeEnvironment normalizes the supplied Environment such that two
  // messages with identical meaning will have identical representation.