Overpass turbo: ‘complete’ statement

I have troubles understanding the complete statment. I read the description on https://wiki.openstreetmap.org/wiki/Overpass_API/Overpass_QL#The_block_statement_complete but I still do not get it.

Here, an example: http://overpass-turbo.eu/s/1c3K; my goal is to get a map of river and its tributaries. When I leave the complete block commented out, I receive the river mapped as relation and a couple of way segments that belong to its tributaries. Works just as expected, here, a picture:

However, when I am commenting out line 12 and using the complete block instead, I only receive the river relation .r which I started with. My set .add has no tributaries and the result looks like this:

Something I am doing wrong ;) I thought the complete block would iterate several times, thus, adding more and more segments of contributing rivers and streams. Thanks for some hints!