Unable to create specify a simple three-generation pedigree

[nh13]

I'd like to specify which reproducing parent from a branch in previous generation to use, but we only get the first one.

If two parents are listed as in [parentbranch1][parent_branch2], the two reproducing parents in the indicated branches (from the previous generation) have children.

For example, I want:

But the closest def I could create was:

def example 1 3
1 1 3
2 2 3 1:1 2:2 3:3
3 2 2 1:1_2 2:2_3

which gives:

[williamslab]

Only one person per branch is allowed to reproduce. This is the reason that it's feasible to create multiple branches: if you have six branches in generation 2, you can get the mating structure you want.

[nh13]

Here's the def file for any future individual:

def example 1 3
1 1 3
2 1 6
3 2 2 1:2_3 2:4_5