How do I obtain the matrix ordering number of the boundary nodes?

In the following example I have generated a hexagonal mesh and six boundary nodes.

I want to obtain [1,4 0, 6, 5, 2] as an array.

I might be able to detect the matrix elements where the values are tgv.

```
border C1(t=0,2*pi) {x=cos(t); y=sin(t);label=1;}
mesh Th=buildmesh(C1(6));
plot(Th,wait=false);
fespace Vh(Th,P1);
Vh phi, w;
varf Prob(phi,w) = int2d(Th)(dx(phi)*dx(w)+dy(phi)*dy(w)) + on(1,phi=0);
matrix A = Prob(Vh, Vh);
cout << A << endl;
```