wierd problem here, line 6 and line 11, is wierd, I can't figure it out why?

```
1 l1 = ['a', 'b', 'c']
2 
3 l2 = [[]] * 3
4 for i in xrange(0, len(l1)):
5     l2[i%len(l1)].extend(l1[i]) # look! not [li[i]] here
6 print 'l2: ', l2  # problem is here
7 
8 l3 = [[]] * 3
9 for i in xrange(0, len(l1)):
10     l3[i%len(l1)].extend([l1[i]]) 
11 print 'l3: ', l3
12 
13 l4 = [[]] * 3
14 for i in xrange(0, len(l1)):
15     if l4[i%len(l1)] == []:
16         l4[i%len(l1)] = [l1[i]]
17     else:
18         l4[i%len(l1)].extend([l1[i]])
19 print 'l4: ', l4

```

output blow:

```
l2:  [['a', 'b', 'c'], ['a', 'b', 'c'], ['a', 'b', 'c']]
l3:  [['a', 'b', 'c'], ['a', 'b', 'c'], ['a', 'b', 'c']]
l4:  [['a'], ['b'], ['c']]

```

Someone can point out why? Thanks.