'Yields all concepts that are properties (i.e., adjectives).
For example: "cold is-property-of winter" => "cold".'

@property
def properties(self):

    if (self._properties is None):
        self._properties = (e for e in self.edges if (e.context == 'properties'))
        self._properties = set(chain(*((e.node1.id, e.node2.id) for e in self._properties)))
    return self._properties
