Given this vector

```
a = [1 2 3 4]

```

I want to create a matrix like this

```
b = [1 0 0 0;
     2 1 0 0;
     3 2 1 0;
     4 3 2 1;
     0 4 3 2;
     0 0 4 3;
     0 0 0 4]

```

in a vectorized way not using loops.