What should `nodes` of a relation return?

[clarisma]

Currently, relation.nodes returns an empty set.

It may make sense to instead return a set of all nodes that make up the geometry of the relation. If so, in what order should they appear? way.nodes may contain duplicates (an ordered list rather than a set), but what would make sense for relations?

Possible use cases:

1. Find all border crossings of a country's area relation
2. Count the number of traffic lights encountered by a bus route

• For case 1, an unordered set of nodes would suffice.

• For case 2, traffic-light nodes should appear multiple times if the route traverses a segment more than once (but what about traffic lights at the end points of two connected ways? )

To stay consistent, node.nodes should then return the node itself.