DerivationID merging

[LucPrestin]

As a user, I'd like the derivation IDs to be merged meaningful instead of just concatenating them so that they can be properly used for future merges.

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Hints & Thoughts

1. Find parent
2. Copy everything until (including) parent to result
3. Copy derivation IDs of one child from the parent on to the result
4. Copy derivation IDs of the other child from the parent on to the result

Example:
[a, b, c, d, e] + [a, b, c, f, g, h] = [a, b, c, d, e, f, g, h]