Cabin travels through exactly one Shaft
Shaft is conduit for exactly one Cabin
A Cabin moves up and down its Shaft to deliver passengers from one Floor to the next. But imagine the following case where we split a building vertically into two Banks covering two Shafts.
It’s tempting to consider the possibility of a Shaft containing more than one Cabin. A simpler formulation, however, is to say that we have four Shafts two upper Shafts stacked on two lower Shafts. So by definition, a Shaft really does enclose the travel path of only one Cabin.
The case where it might be useful to think of a multi-Cabin Shaft in some future release would be a situation where two Cabins actually service the same set of Floors, but there is no requirement in this release to address the complexity of such an arrangement.
An empty Shaft is of no interest or utility in our system so a Shaft is defined to contain exactly one Cabin.
Cabin.Shaft -> Shaft.ID