@property
def hexagonal_edges(self):
    'Gets the three half-edges on the hexagonal boundary incident to a black node and point in ccw direction.'
    first = self.half_edge
    res = [first]
    second = first.opposite.next.opposite.next
    res.append(second)
    third = second.opposite.next.opposite.next
    res.append(third)
    for he in res:
        assert (he.is_hexagonal and (he.color is 'black'))
    return res
Gets the three half-edges on the hexagonal boundary incident to a black node and point in ccw direction.
@property
def hexagonal_edges(self):
    
    first = self.half_edge
    res = [first]
    second = first.opposite.next.opposite.next
    res.append(second)
    third = second.opposite.next.opposite.next
    res.append(third)
    for he in res:
        assert (he.is_hexagonal and (he.color is 'black'))
    return res