>>> from graf_exp import *

>>> 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)
>>> vars_mes_usades(g)
['b', 'c']