Equivalent of emptymesh in 3D

General Discussion
1

Hello Freefemers,

Emptymesh command is very useful to handle boundary values in 2D. For example

mesh Th=square(10,10); 
fespace Vh(Th, P1);
Uh      u, uh;
mesh    Thb=emptymesh(Th);
fespace Uhb(Thb, P1);
Uhb     ud=x, un=y;
problem laplace(u,uh) 
= int2d(Th)( u*uh+dx(u)*dx(uh)+dy(u)*dy(uh) )
- int1d(Th,1,2) (un*ub)
+ on(3,4, u=ud);

I am looking for an equivalent of emptymesh functionality in 3D. From the documentation, I would expect that the extract command may provides the same functionality. Can anyone confirm this, and eventually provide any piece of code how to implement it?

Thank you!