code: def _combined_grouping_values(grouping_name,collection_a,collection_b):
      
      new_grouping= collection_a.groupings.get(grouping_name,{}).copy()
      new_grouping.update(collection_b.groupings.get(grouping_name,{}))
      return new_grouping
docs: returns a dict with values from both collections for a given grouping name
  
      Warning: collection2 overrides collection1 if there is a group_key conflict