How to get external boundary from multiple overlapping meshes?

General Discussion
1

I’m generating individual meshes for rectangular pipes (using square(...)) and combining them with +. I’d like to extract the external boundary of the union of these meshes to create a mesh without internal overlaps or duplicated boundaries. Is there a recommended way to do this, or do I need to reconstruct everything from border definitions?

real L = 10.0;            // vertical pipe
real H = 8.8;             // horizontal pipe
real framed = 0.0635;     // frame pipe dia. (2.5 in)
real branchd = 0.0254;    // branch pipe dia. (1 in)
real spacing = 2.5;       // distance between branch centers
real offset = 1.25;       // distance from top-bot to first-last branch

int nbranches = int((L - 2*offset)/spacing) + 1;

func mesh addrect(real a, real b, real w, real h) {
    int nx = 10;
    int ny = 10;
    return square(nx, ny, [a + w*x, b + h*y]);
}

mesh Th = addrect(0, 0, H, framed); // bottom
Th = Th
    + addrect(0, L - framed, H, framed)   // top
    + addrect(0, 0, framed, L)            // left
    + addrect(H - framed, 0, framed, L);  // right

for (int i = 0; i < nbranches; ++i) {
    real ypos = offset + i * spacing;
    Th = Th + addrect(0, ypos - branchd/2, H, branchd);
}

plot(Th, wait=1, cmm="Piping mesh");