In https://wiki.openstreetmap.org/wiki/Overpass_API/Overpass_API_by_Example
there is an example as shown below in the code section. I would like to know what is
b
u
and b.u?
Code:
[timeout:600];
rel[admin_level=2][wikidata]->.a;
foreach .a -> .b (
rel.a(if:t[wikidata] == b.u(t[wikidata]))->.d;
rel.d(if: id() == d.min(id()) && d.count(relations) > 1 );
convert rel ::id = _.u(id()),
wikidata = _.u(t[wikidata]),
all_relation_ids = d.set( id() );
(._ ; .result; ) -> .result;
);
.result out;