`assert str((X*Y)[4,2]) == "Sum(%s(4, _k)*%s(_k, 2), (_k, 0, m - 1))" % (X.name, Y.name)` or will this work:

```
assert (X*Y)[4,2].dummy_eq(Sum(X[4, k]*Y[k, 2], (k, 0, m - 1)))
```