Flickr supports machine tags

Photoservice Flickr announced support for the so-called "machine tags" , that is, tags with several sublevels. Innovation focused mainly on developers.

Machine tags (also called triple tags) consist of three parts. The first, the “namespace” (namespace), determines which class the tag belongs to. The second, known as the “predicate”, indicates the properties possessed by the first part. The third is the meaning. The functionality is provided by the Flickr API.

The site has previously allowed informal use of machine tags, for example, “geo: lon =” and “geo: lat =”. Now any restrictions on syntax are removed.
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flickr: user = straup
flora: tree = coniferous
medium: paint = oil
geo: cartier = "plateau mont royal"
geo: neighbor = geo: cartier