old_contents: from .hdbscan_ import HDBSCAN, hdbscan
  from .robust_single_linkage_ import RobustSingleLinkage, robust_single_linkage
  
new_contents: from .hdbscan_ import HDBSCAN, hdbscan
  from .robust_single_linkage_ import RobustSingleLinkage, robust_single_linkage
  from .validity import validity_index
  
  
subject: Add validity_index to default imports.