sympy matrix to explicit sum and back (to matrix notation)

Question

I am working in sympy with symbolic matrices.

Once made explicit I can not return to implicit representations.

I tried to work something out with the pair of .as_explicit() and MatrixExpr.from_index_summation(expr)

But the latter seems to expect an explicit sigma notation sum, not a sum of indexed elements.

As a minimal working example here is my approach on matrix multiplication:

A = MatrixSymbol('A',3,4)
B = MatrixSymbol('B',4,3)
Matrix_Notation = A * B
Expanded = (A * B).as_explicit()
FromSummation = MatrixExpr.from_index_summation(Expanded)

Here we can see, that FromSummation is still the same as Expanded

I suppose that the Expanded expression should be converted to sigma sums such that .from_index_summation can be expected to work. But how can this be done?