check
Whether the space-exclusivity invariants are checked at the end of every instant in which the guide path did anything.
Off by default, because the check walks every zone and every transporter and a model that is correct pays for nothing. Tests turn it on: it is the standing proof that no transporter ever shares space with another, ever covers a broken run of zones, or ever loses track of what it holds.
The initial value comes from the CHECK_INVARIANTS_PROPERTY system property, so that a whole suite -- or a whole model tree being debugged -- can be checked without every system in it being found and switched on by hand. Setting this explicitly still decides for itself.