Interpolation with zero extension

interpolate from a small mesh Ths to a larger mesh Th, for example

fespace Vh(Th, P2);
Vh u;

fespace Vhs(Ths, P2);
Vh us=sin(x+y);

In default, u=us would extend the value of us to the whole Th, how can we let u=0 on Th-Ths (zero extension)?