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

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