[Tagging] Mapping a negative

[Kytömaa Lauri]

>Are there any other OSM conventions that indicate a lack of a facility?  Maybe:
>    toilets=no

Not going into the wiki-approved new schemes, currently many highway=bus_stop
nodes have one or more of: 
 shelter=no (54800)
 bench=no
 timetable=no
 waste_basket=no
 departures_board=no
Each of these could, at least in some sense, be a separate facility/amenity,
in contrast to for example unaccepted payment options, e.g. payment:coins=no.

Also, in places where a pedestrian is legally obliged to use the painted crossing
"nearby", at a point where a footway way crosses a way depicting the road, but 
where there is no crossing, there can be (1971 uses) the tag crossing=no.