TrackTriggerMoversTrait
constTrackTriggerMoversTrait:Trait<{entered: () =>Entity[];inside: () =>Entity[]; }>
Which movers a track trigger holds: each one standing in it, and each
one that came in this step, a second while the first stands in it
included. The step adds it beside TrackTriggerTrait and removes it
once that is gone. dumpState names each mover by its dump key.