sympy/sympy — pull request #1467 by mrocklin (2012-08-06 23:20:11+00:00)
Title: ImmutableMatrix defaults to MatrixBase operators

I.e.

```ImmutableMatrix.__add__ = MatrixBase.__add__```
instead of
```ImmutableMatrix.__add__ = MatExpr.__add__```

This causes ``ImmutableMatrix * ImmutableMatrix => ImmutableMatrix```
instead of ```ImmutableMatrix * ImmutableMatrix => MatExpr```

I.e. ImmutableMatrices evaluate by default.