Child faces grouped by parent - script

I was using Loop subdivision, and needed to access groups of faces in the subdivided mesh according to the face in the original mesh they descended from, so I made a simple little script making use of the way Weaverbird orders its output to extract this. I'm sharing it here in case anyone else finds it useful:

It takes as input the original and subdivided meshes, and an index of a face in the original mesh. The output is a joined mesh of all the corresponding faces in the subdivided mesh.