Documentation flow issue
CHAPTER 6
ESSENTIAL BASIC FUNCTIONALITY

As the reader moves from section 6.1 to 6.2 to 6.3.1

The df variable does not trace through as  the tutorial would expect.

I think it needs to insert something like the following 

```
In [14]:

d = {'one': Series(randn(3), index=['a', 'b', 'c']),'two': Series(randn(4),index=['a','b','c','d']),'three': Series(randn(3), index=['b','c','d'])}
df =DataFrame(d)
```
