Matrix.Expand($M$, $n$, $x$) takes a quadratic matrix $M$ of dimension $m$ by $m$ and enlarges the dimension by $n$ to $n+m$ by $n+m$. The new diagonal values are set to $x$. If $x$ is not given all new entries are set to 0.
A = Matrix.New({{1,2}, {3,4}}) B = Matrix.Expand(A,4) C = Matrix.Expand(A,4,3) print(B) print(C)
{ { 1 , 2 , 0 , 0 } ,
{ 3 , 4 , 0 , 0 } ,
{ 0 , 0 , 0 , 0 } ,
{ 0 , 0 , 0 , 0 } }
{ { 1 , 2 , 0 , 0 } ,
{ 3 , 4 , 0 , 0 } ,
{ 0 , 0 , 3 , 0 } ,
{ 0 , 0 , 0 , 3 } }