Matrix addition: 2x2 + 2x3

[parisse]

The design philosophy of giac is: the user knows what he does, it's important to provide features to advanced users.
If you have 2 lists of different sizes to add, you can
a/ forbid addition
b/ add them by adding 0 at the right of the shortest
c/ add them by adding 0 at the left of the shortest
On the Prime, if you add two matrices with different sizes that are tagged as matrices a/ happens. For example matrix[[1,2],[3,4]]+identity(3)
Otherwise and in Xcas, b/ applies if the list are tagged as 1-dimensional polynomials (poly1[1,2]+poly1[1,2,3]) because coefficients are written in decreasing order and c/ applies otherwise.