A press room sends a proof out and gets it back three days later with a pencil note in the margin. To pick the job up again it has to know which run the sheet belongs to, which plate the page was set for, and which ink the colour bar was drawn against — and none of that is anything the person holding the sheet should be shown. So it travels under the document instead of on it: the file carries the run, the page carries its plate, and the colour bar, placed twice on that page, carries the ink it was set against.
Each of those three is filed under the name of the program that left it, so two programs leaving notes in the same document never read each other's, and each is stamped with the moment its program last touched it. None of it is drawn, and no reading software shows any of it: what is on the page is a proof, and what is under it is how the press room finds its way back. It rides along with the file wherever the file goes, and is waiting when the program that wrote it opens the document again.
Nothing is left out and nothing is simplified: this is the program exactly as it runs, for the developer who uses this library. The third is the description itself: a plain list of what the page has to say, which is what the service is sent.
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An invoice: each billed line follows the one before, the totals sit under the last, and the QR code holds the transfer. The customer points a telephone at it, and the banking app offers the exact sum into the exact account, with nothing retyped.
The note that travels with the goods: a barcode against each article, a box the receiver ticks, and the consignment's own code.
A payslip: eighteen contribution lines, with the employee's share and the employer's, and for each the month's amount then the total since January. Each figure is tied to the line and the column it stands under, for software that reads the page aloud.
A services agreement over two pages where no clause is ever cut in half by a page turn, ending in signature, name and date fields filled in on screen.
A certificate whose border, ribbon and seal are drawn as lines rather than photographed: sharp at any size, and a fraction of the weight.
A boarding pass carrying, as a QR code, the one line the gates at an airport read — one character out of place and it is refused.