Fig. \[fig:1\]. When a sliding path exits at a unit cell boundary, the underlying lattice periodicity demands that it reenters on the opposite side. In Fig. \[fig:1\] we show two periodic paths with 0 and 30 degrees, respectively. The first (red path) in Fig. \[fig:1\] dissects the unit cell once along the Cartesian x-axis before returning to its starting point, whereas the second one (blue path) in Fig. \[fig:1\] cuts the unit cell twice. As can be seen from Fig. \[fig:1\] the energy variations along these two paths are distinctly different representing two extremal cases. Additional to these periodic paths we also consider 10 quasi aperiodic ones, namely for $-33$, $-29$, $-15$, $-10$, $3$, $5$, $10$, $21.13$, $45$, and $50$ degrees. We note that due to the inevitable representation of irrational numbers by rational fractions in a computer these paths are essentially also periodic, however, with very long periods. The forces acting on the upper slab have a parallel (friction) and an in-plane perpendicular (reaction) component with respect to the direction of the path. Since our model allows for both positive and negative contributions to the friction force, we use the convention that friction forces pointing against the sliding direction 