Not much happens. I must be doing something wrong - or it is not possible.
I am asked for:
1) 1.st group of beams - I choose the two angled beams.
2) 2.nd group of beams - I choose the two horizontal beams.
3) 1.st reference beam - I choose the lower angled beam.
4) 2.nd reference beam - I choose the one horizontal beam.