yttrium

>>> def tree(func=None, level=-1):
...     if not func:
...         func = tree
...     if not level:
...         return func()
...     return defaultdict(lambda : tree(func, level-1))
...
>>> f = tree()
>>> f
defaultdict(<function <lambda> at 0x2b00e978>, {})
>>> f[3]
defaultdict(<function <lambda> at 0x2b00eb28>, {})
>>> f[1][2][3][4]
defaultdict(<function <lambda> at 0x2b00ec48>, {})
>>> f = tree(float, 3)
>>> f[1]
defaultdict(<function <lambda> at 0x2b00eb28>, {})
>>> f[1][2]
defaultdict(<function <lambda> at 0x2b00ec00>, {})
>>> f[1][2][3]
0.0

reduce(lambda x,y: (x[0],x[0].__setitem__(y[0],x[0][y[0]].union(y[1]))),map(lambda x:(re.match(r'(www\.)?(.*)',x[0]).group(2),x[1]),l),(defaultdict(set),))[0].items()