“…The approach used to compute the domain integrals is to adopt internal cells, see for example [2,9,14,16,17,21,[26][27][28][29][30]33,[35][36][37][38][39][40][41][43][44][45][46]50,[53][54][55][56][57][59][60][61]63,65,66]. These cells are like finite elements in appearance, but the main difference is that they do not introduce any additional unknowns to the problem.…”