Writing a custom propagator is worth considering when one constraint carries much of a problem's structure. My paper Propagation Algorithms for the Minimum-Distance Constraint over Selected Points, to be presented at ModRef 2026, works through one such case in Gecode.

Suppose we want to select five facility locations, spread out so that the closest pair is as far apart as possible. Let z denote that minimum distance. A straightforward CP model introduces a distance variable for every selected pair, then constrains z to be the minimum of those distances. This is a clear specification, but it creates quadratically many auxiliary variables and propagators.