parent pedigree IDs maybe don't need to be in the individual metadata

[bhaller]

@petrelharp and I were puzzling about this. Why are these pedigree IDs there exactly? Could they be reconstructed from the parents column? (I think they are used to make the parents column, but maybe that could be done differently...) Are they needed/useful? If they're not, it'd make the metadata smaller to get rid of them.

[gregorgorjanc]

If parents are in the parent column of the individual table, then storing them also in metadata is indeed duplication.

[bhaller]

If parents are in the parent column of the individual table, then storing them also in metadata is indeed duplication.

Yes; it isn't quite that simple, since parents might get simplified away. For example, if we toss the parental pedigree IDs and some parents get simplified away, we can no longer calculate the relatedness between individuals in SLiM after reloading from the saved tree sequence. I suspect that this issue will get closed in the end; Peter and I just want to better understand why we made the decisions we made in this design. :->