  We propose a simple model of protesters scattered throughout a city who want
to gather into large and mobile groups. This model relies on random walkers on
a street network that follow tactics built from a set of basic rules. Our goal
is to identify the most important rules for fast and robust flocking of
walkers. We explore a wide set of tactics and show the central importance of a
specific rule based on alignment. Other rules alone perform poorly, but our
experiments show that combining alignment with them enhances flocking, and that
obtained groups are then remarkably robust.
