Collect all items into a single set per section.

def get_section2items(self, itemkey):
        """Collect all items into a single set per section."""
        sec_items = []
        section2usrnts = self.get_section2usrnts()
        for section, usrnts in section2usrnts.items():
            items = set([e for nt in usrnts for e in getattr(nt, itemkey, set())])
            sec_items.append((section, items))
        return cx.OrderedDict(sec_items)