docs: def _invertMapping(mapping):
      """Converts a protein to peptide or peptide to protein mapping.
  
      :param mapping: dict, for each key contains a set of entries
  
      :returns: an inverted mapping that each entry of the values points to a set
          of initial keys.
      """
      invertedMapping = ddict(set)
      for key, values in viewitems(mapping):
          for value in values:
              invertedMapping[value].add(key)
      return invertedMapping