How to "restart" a matrix from an especific element

Hi everyone. I have a square matrix like this: [1,2,3;4,5,6;7,8,9]

How could I restart the sequence from an especific element? For example, I want to restart the sequence from 4, so the matrix will be: [1,2,3;4,1,2;3,4,5]

Thanks a lot!

Answer 1

[Walter Roberson]

restart_after := 4;
Matrix(restart_after+1:end) = Matrix(1:end-restart_after);

[Facundo Beer]

Thanks. But that works when the matrix is ordered by columns. If the sequence is sorted by rows, like the example, don't.