Despite the failure of overly coarse grids, remarkably, Demeler's 2DSA algorithm consists exclusively of repeat applications of such coarse grids: They are considered ''subgrids'' of a hypothetical grid with much higher resolution, which is never actually completely fitted to the data, but nevertheless suggested to reflect the final resolution of the distribution. The details are not entirely clear, but there are two key ideas: (I) The coarse grids are translated relative to each other multiple times by increments D 2 s and D 2 f r , and their results are joined. (II) The joined set of grid points with inactive nonnegativity constraints from (I) is used to form a new, second-stage irregular grid of similarly low number of grid points as the initial grid. 4 The Demeler scheme of repeat application of different coarse subgrids, storage, and combination of their results, is termed a ''divide-and-conquer'' strategy. Divide-and-conquer algorithms are well-known tools in numerical mathematics that facilitate the use of parallel computation to solve problems, such as singular value decomposition (Arbenz and Golub 1988; Gu and Eisenstat 1995; Xu and Qiao 2008) . Generally, such algorithms are established by proof of their correctness. This criterion has not been attempted for the 2DSA algorithm. Concerns arise from the following arguments: