Dynamical family-sorting procedure. (q + a) vs. (Q − a) for all tracers, corresponding to the smallest size bin (β = 0.44), which are present in one given spatial cell. We plot the tracers present at the beginning (init tracers) of a collisional time step plus the virtual tracers that are temporarily created as a result of mutual tracer-tracer collisions amongst larger size bins, and the final tracers that are eventually kept at the end of the sorting and selection procedure. The thick black lines show the two dynamical categories identified by the sorting algorithm (see Sect. 3.7). The set-up is that of the pedagogical test run, of an unperturbed disc initially confined to an inner ring of parent bodies, presented in Sect. 3.10. The considered spatial cell is located 8 AU outside the inner birth ring.