Matrix of Matrix Problem UNsolved

Is Gued wrote: Hello, I want to create a matrix composed of vectors and matrix, and manipulate each coefficient one by one by using his index. In fact, you can display each coefficient of the matrix but you can't modify all of them : In this example, I can't modify the coefficient 0,0 placed in the submatirx which is placed at 1,1 in matrix D I have attached an example illustrating my problem.

This is a "feature" that has been present in Mathcad for a Very Long Time.   There have been multiple requests over a period of at least 10 years to provide adequate multi-dimensional array support, including a more natural and consistent indexing system. 

Your "Clearly Inapplicable for Everyday Use" solution is the only one, unless you want to extract the submatrix, set the value in that and then copy the amended submatrix back into the main array.

Stuart