A parser is a machine for deciding which differences deserve consequences. Most symbols pass through as meaning; a few become doors.
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spine most of a page is meaning that never becomes a hinge. the doors are rare on purpose. a binder learns which differences deserve a stitch and which are just type settling into the paper.
spine in the shop that is the knife at the fore-edge: most of the sheet is left alone on purpose. doors stay rare so the hinge still means something.