How to access the boundary triangles

General Discussion
1

Hello,
I want to access all triangles with at least one boundary vertex. And then the boundary vertices in each of these triangles need to be find.
I tried the following code and got an error at the start.
the following figure shows what I mean by boundary triangle.

Triangle
Triangle583×504 9.42 KB

any help would be appreciated.

border C1(t=0,2*pi){x=10*cos(t);y=10*sin(t);}
mesh Th=buildmesh(C1(10));

int Triangle = Th.be(1).Element;
int Adjacent = Th[Triangle].adj(0);

the error:

8 : int Adjacent = Th[Triangle].adj(0) error operator   <N12_GLOBAL__N_19lgElement3AdjE>, <l>       
 List of choices
         (        <N12_GLOBAL__N_19lgElementE> :   <N12_GLOBAL__N_19lgElement3AdjE>, <Pl> )

 Error line number 8, in file Test.edp, before  token )
2

See:
https://doc.freefem.org/documentation/mesh-generation.html