01 The yard
A container yard is a queue with a shape.
Boxes do not move because a truck arrived. They move because a slot, a crane, and a dock door line up in that order. When one of the three slips, the yard does not show an error: it shows a stack that is one row taller than yesterday.
The useful question is not how many boxes are in the yard. It is which boxes have a slot but no crane, and which have a crane but no slot.