The crate that wouldn’t quite close
In a dim cargo shed by the water, I eased a delicate instrument into a foam-lined crate. A strap inside was stitched to a little moisture pack on the wall. As the lid came down, I felt a tug. The instrument only sat safely in certain positions.
That crate is the whole idea. The instrument is the tiny thing we care about, the moisture pack is its surroundings, and the strap is a pre-made link between them. With that strap fixed, not every starting position is even possible. Only the ones that let the lid close count.
With no strap, the handling rule is simple. Any safe start turns into a safe finish after the journey. With the strap, the finish looks like the usual result plus an extra shove that comes only from that built-in link, while still obeying the “lid must close” limit.
The new trick is neat bookkeeping. They take that “usual rule plus shove” and turn it into one clean, straight-line rule you can apply to any description you might scribble down, even the odd ones. In crate terms, it’s like adding one correction that scales with how much “stuff” the tag claims. Takeaway: one fixed strap gives one fixed correction, and there’s only one consistent way to extend it.
But there’s a catch. The extended rule behaves perfectly on the starts that really fit, like sensible packing. Feed it an impossible start and it can spit out something no real crate could hold. The maths stays tidy, but the result might not describe anything you could actually have.
They also write a moment-by-moment update, as long as the clean, no-strap motion can be run forwards or backwards at that instant. The strap adds a new term, but it doesn’t change the smooth turning part. It only changes the messy part, like scuffing, damping, and little leaks during the trip.
In a simple example, the “allowed starts” shrink as the strap gets stronger, from a whole range down to almost a single choice. The strap gives a repeating extra push to some parts of the motion, while leaving others alone in that setup. I looked back at the crate and stopped fighting the tug. The strap was never a detail, it was the boundary.