>>> import networkx as nx
>>> g = nx.DiGraph()
>>> gent = ['Joan','Anna','Pere','Josep','Maria','Carla','Rut','Gisela']
>>> fills = [ ('Joan','Anna'), ('Joan','Pere'), ('Joan','Josep'), ('Maria','Anna'), ('Maria','Pere'), ('Carla','Josep'), ('Pere','Rut'), ('Pere','Gisela') ]
>>> g.add_nodes_from(gent)
>>> g.add_edges_from(fills)

----

>>> import ex3
>>> ex3.descendents(g, 'Maria') == {'Pere', 'Gisela', 'Rut', 'Anna'}
True
>>> ex3.descendents(g, 'Pere') == {'Gisela', 'Rut'}
True
>>> ex3.descendents(g, 'Anna') == set()
True