docs: def modSymbolsFromLabelInfo(labelDescriptor):
      """Returns a set of all modiciation symbols which were used in the
      labelDescriptor
  
      :param labelDescriptor: :class:`LabelDescriptor` describes the label setup
          of an experiment
  
      :returns: #TODO: docstring
      """
      modSymbols = set()
      for labelStateEntry in viewvalues(labelDescriptor.labels):
          for labelPositionEntry in viewvalues(labelStateEntry['aminoAcidLabels']):
              for modSymbol in aux.toList(labelPositionEntry):
                  if modSymbol != '':
                      modSymbols.add(modSymbol)
      return modSymbols