ambiguous col/index name
This feels vaguely familiar but the DataFrame display is wrong after the stack/reset_index operations below (xref: http://stackoverflow.com/questions/14916358/reshaping-dataframes-in-pandas-based-on-column-labels/14917572#14917572):

```
In [58]: df
Out[58]: 
sample          s1                  s2          
variable         x         y         x         y
0         0.298171 -1.336739  1.117346  2.291124
1        -1.802796 -0.154719  0.532947 -0.338009
2         0.086519  1.641511  0.001948 -0.575835
3        -1.638318 -0.206623  0.238649 -0.145120
4         0.368954  0.654914  1.529368  0.300910
5         1.609236 -0.418993 -1.088934  0.912224
6         0.437332  2.668343  0.751687  0.289443
7         0.627601  0.057554  0.085994  2.038858
8         0.785895  1.695244  0.085235  1.017542
9         0.998866  1.001776 -0.662358  0.280511

In [59]: df.stack(0).reset_index(1)
Out[59]: 
variable sample         x         y
0            s1  0.298171 -1.336739
0            s2  1.117346  2.291124
1            s1 -1.802796 -0.154719
1            s2  0.532947 -0.338009
2            s1  0.086519  1.641511
2            s2  0.001948 -0.575835
3            s1 -1.638318 -0.206623
3            s2  0.238649 -0.145120
4            s1  0.368954  0.654914
4            s2  1.529368  0.300910
5            s1  1.609236 -0.418993
5            s2 -1.088934  0.912224
6            s1  0.437332  2.668343
6            s2  0.751687  0.289443
7            s1  0.627601  0.057554
7            s2  0.085994  2.038858
8            s1  0.785895  1.695244
8            s2  0.085235  1.017542
9            s1  0.998866  1.001776
9            s2 -0.662358  0.280511
```
