'Returns the top properties in the concept halo, sorted by betweenness centrality.
The return value is a list of concept id\'s instead of Concepts (for performance).'

@property
def properties(self):

    if (self._properties is None):
        g = self.graph.copy(nodes=self.halo)
        p = (n for n in g.nodes if (n.id in self.graph.properties))
        p = [n.id for n in reversed(sorted(p, key=(lambda n: n.centrality)))]
        self._properties = p
    return self._properties
