As I'm working on rivers and their basins I thought I might try to just add child relations to a parent relation - but it did not render!

Example:
Take Lena river,
1. make a relation for the whole length of the river
2. find some tributaries (Vitim, Kirenga, Olyokma) and repeat step 1
3. make a river basin relation and add relations Lena, Vitim, Kirenga, Olyokma
4. test it

It did not render. It only renders when adding all single ways of all rivers (1).

(1) http://www.openstreetmap.org/?relation=166216