a list of indices in the faceOrders array which are now invalid due to now being two faces on the opposite sides of the split line.
This method is meant to accompany foldGraph, or any operation which divides many overlapping faces in the same graph. During the splitting splitFace() creates a bunch of new faceOrders from all the old faces corresponding to the new faces (new faces count twice as many as old). This creates a bunch of faceOrders between faces which no longer overlap and can be easily calculated since we know the dividing line, compare the center of the faces to find which side of the line they lie, for face pairs which lie on opposite sides, return this index in the faceOrders array.
a FOLD object with vertices_coords in the same state when the split occured (in most cases this is the folded form vertices).