2D boundary of a cube

Hi there, I’ve constructed a cube in such a way

load “msh3”
int[int] ll= [1,2,1,2,3,4];
mesh3 Th = cube(20,20,20,label=ll);

how do I introduce a 2D mesh defined as one of the faces of the cube? Is it possible to use meshL Th1=buildmeshL(…) even if I’ve not used borders to construct the cube?


I solved with

int[int] llabs = [5];
meshS ThS = extract(Th,label=llabs);