code: func validLocationStorageClass(location, storageClass string) error {
  	if _, ok := GCSStorageClass[storageClass]; !ok {
  		return ErrUnknownStorageClass
  	}
  
  	if storageClass == regional {
  		if _, ok := GCSMultiRegionalLocations[location]; ok {
  			return ErrBadLocationStorageClass
  		}
  		return nil
  	} else if _, ok := GCSStorageClass[storageClass]; ok {
  		if _, ok := GCSRegionalLocations[location]; ok {
  			return ErrBadLocationStorageClass
  		}
  		return nil
  	} else {
  		return ErrUnknownStorageClass
  	}
  }
docs: // validDurability returns nil error on valid location-durability combination
  // and non-nil explanation on all else.