I want to know how to implement this expression:

First what is \tau_1

but you can do some think like

```
mesh Th=square(1à?10);// def of Omega_h
fespace Ph(Th,P0); // to define tau1 constant by triangle
fespace Vh(Th,P1) ; // for ph and ph star
Ph tau = 1 ; // here def value tau
macro grad(u) [dx(u),dy(u)]//EOM
varf vexp(p,pp) = int2d(Th)( grad(p)'*grad(pp)*tau) ;
matrix Mat = vexe(Vh,Vh); // matrix associated to Bilinear form vexp
```