A *readout* cursor -- a marker a host keeps for something it computed, rather than an instrument the user placed -- has to be told apart from its neighbours, and the cursor API could set everything about one except its colour. The setter keeps the id and ignores a stale one, like the setters beside it. Tested by `a_cursor_can_be_recoloured_by_id`, and the suite is 356 cases, 0 failed.