Further properties of the dynamical clock A+ indicator in a toy model of
  pure dynamical friction
  The Alessandrini A$+$ indicator is a quantitative measure of star cluster
dynamical evolution based on the mass-segregation of blue straggler stars. A$+$
is defined as the integral of the cumulative distribution of blue straggler
stars with the radius measured in logarithmic scale, minus a term related to
the reference population used. In a companion paper I introduced a simplified
model of dynamical friction and calculated the A$+$ indicator analytically.
Here I show further properties of the time evolution of A$+$, focusing in
particular on the physical interpretation of its time derivative $d{A^+}/dt$. I
find that $d{A^+}/dt$ is the mean of the reciprocal dyamical friction
timescale, weighted by the density of blue straggler stars. I show that it is
non-negative (as expected based on monotonicity) due to the density of
blue-straggler stars being non-negative and that, for a dynamical friction
timescale that is non-decreasing with radius, $d{A^+}/dt$ is also
non-decreasing with time, making the A$+$ indicator a convex function.
