sympy/sympy — pull request #1787 by mrocklin (2013-02-15 17:40:42+00:00)
Title: slice of MatrixSlices simplify

Reduces slices of slices

```Python
>>> from sympy import MatrixSymbol
>>> X = MatrixSymbol('X', 10, 10)
>>> X[:, 1:5][5:8, :]
X[5:8, 1:5]
>>> X[1:9:2, 2:6][1:3, 2]
X[3:7:2, 4]
```