In r1011, some improvements have been made to definition variables. Definition variables can now be used inside algebra expressions. Additionally, definition variables inside of MxMatrices will populate to the 1st row before conformability checking. What this means in plain English: you do not need to specify the starting values for definition variables.
The next objective is to provide the same expressiveness of the square-bracket operator for MxMatrix labels that is currently afforded to algebra expressions. This might take a little longer to implement.