code: def _merge_metadata_dictionaries(cls, dictionaries):
          
          # three levels of nested dictionaries!
          #   {source name: {variant: {attribute: value}}}
          combined_dictionary = {}
          for source_to_metadata_dict in dictionaries:
              for source_name, variant_to_metadata_dict in source_to_metadata_dict.items():
                  combined_dictionary.setdefault(source_name, {})
                  combined_source_dict = combined_dictionary[source_name]
                  for variant, metadata_dict in variant_to_metadata_dict.items():
                      combined_source_dict.setdefault(variant, {})
                      combined_source_dict[variant].update(metadata_dict)
          return combined_dictionary
docs: Helper function for combining variant collections: given multiple
          dictionaries mapping:
               source name -> (variant -> (attribute -> value))
  
          Returns dictionary with union of all variants and sources.