code: def _findUniqueMappingValues(mapping):
      
      uniqueMappingValues = set()
      for entries in viewvalues(mapping):
          if len(entries) == 1:
              uniqueMappingValues.update(entries)
      return uniqueMappingValues
docs: Find mapping entries that are unique for one key (value length of 1).
  
      .. Note:
          This function can be used to find unique proteins by providing a
          peptide to protein mapping.
  
      :param mapping: dict, for each key contains a set of entries
      :returns: a set of unique mapping values