For years, while I was a professional plant breeder, I had a list of ideas I wanted to work on, but my employers didn't see any money in them. One of them was winter hardy citrus for the zone I was in. Zone 6.
Now "retired", I'm working on that list of things I wanted to breed. Especially the hardy citrus. Especially mandarins. Yes, I want them for myself. But also there is the challenge.
When I first posted about it here, Kumin posted about his 20,000 seedlings from C-35 citrange. Out of 3,000 zygotic seedlings he got 12 that have come through 3 winters now in Pennsylvania. Same zone as I'm in. I had not believed it could happen in the F2. I had expected to have to backcross to P. trifoliata to recover winter hardiness. So Kumin showed it was much easier than I thought possible.
And here I learned about Ponciris+ which lacks the resin flavor. So what I had thought would take at least a couple of generations had been done for me.
Now the paper I linked to above says, if I understand correctly, that sweetness won't be as hard to get as I had thought.
Nothing against breeding and growing dwarf citrus. That has brought joy to generations of people. But it is not what I'm about. Likewise, Kumin, Ilya, Mikkel, and other who I know less about. I'm sorry to the ones that didn't come to mind just now.