-- store the triangles oriented such that their volume is
-- positive. If T and T-tilde share \<v0,v1,v2\> and v3,v3-tilde
-- point in opposite directions, one of them has to have negative
-- store the triangles oriented such that their volume is
-- positive. If T and T-tilde share \<v0,v1,v2\> and v3,v3-tilde
-- point in opposite directions, one of them has to have negative