Module mesh 

A body of triangles (BodySpec::of_triangles): what a mesh read from a file (STL) is as a B-rep.

The topology comes from the mesh’s own indexing, not from stitching geometry: two triangles share an edge where they share its two corners. Each triangle is a face on its plane. The plane is a patch over a domain the triangle does not decide — the box of every corner of the mesh seen along the axis the plane faces most — so triangles in one plane lie on the very same patch, and BodySpec::merged makes them one face at once.