Rules of selection for the model. Rule#1: the distance d between the centroids of two substructures of the same level at scale s_l needs to exceed the indicated tolerance. Rule#2: as the child (red) lies within its parent (blue), its extent can go beyond the grey area as long as 75% of its area remains inside its progenitor. Rule#3: child (red) centroid lies within its parent (blue), but avoids the forbidden area at the centre (grey). The minimal separation d_min between the centroids of siblings is seen as the diameter of the forbidden area.