>>> from graf_exp import crea_digraf_exps

>>> le = [('a*(b+c)-c/b', 'a*(b+c)'), ('a*(b+c)-c/b', 'c/b'),
... ('a*(b+c)', 'a'), ('a*(b+c)', 'b+c'),
... ('c/b', 'c'), ('c/b', 'b'),
... ('b+c', 'b'), ('b+c', 'c')]
>>> g = crea_digraf_exps(le)
>>> sorted(g.nodes())
['a', 'a*(b+c)', 'a*(b+c)-c/b', 'b', 'b+c', 'c', 'c/b']
>>> for i, e in enumerate(sorted(g.edges())):
...   print(e)
('a*(b+c)', 'a')
('a*(b+c)', 'b+c')
('a*(b+c)-c/b', 'a*(b+c)')
('a*(b+c)-c/b', 'c/b')
('b+c', 'b')
('b+c', 'c')
('c/b', 'b')
('c/b', 'c')