[Tagging] natural=water inside natural=wetland

[Jean-Marc Liotier]

Consider a wetland that contains a water body. I'm used to map that as 
natural=water inside natural=wetland - no multipolygon fanciness, just 
one on top of the other. JOSM validator complains about it, which irks 
me, so I opened a ticket at https://josm.openstreetmap.de/ticket/19171 - 
where mdk suggests that I may be doing it wrong...

Is my simple way incorrect ? It feels correct to me because wetlands are 
complex objects - water bodies are part of them, cross them or partially 
overlap them. From a tagging point of view, it implies that some area is 
both natural=water and natural=wetland - I see no problem with that... 
But others might consider that a logical impossibility.

So, which is the correct way: plain natural=water inside 
natural=wetland, or a natural=water multipolygon with natural=wetland on 
its inner ?