Note that we have for convenience chosen to work with a coordinate coframe i dx, with the imposed constraint √ - N = g. In general, one may of course use an arbitrary spatial coframe, say i θ (x), for which the associated gauge choice reads √ - N = w (x) g, with w (x ) being a density of weight − 1. Such a frame will be used in Section 2.3.1. This general kind of spatial coframe was also used extensively in the recent work [40Jump To The Next Citation Point].