If the `key` is unknown the `at(key)` will throw. So I guess if you'd rather throw than get a 0 its OK. To replicate the same behaviour as before

```C++
auto itFound = flavours.find(jetRef);
unsigned int flavour = itFound != flavours.end()? itFound->second : 0;
jetWithFlavour.second = reco::JetFlavour(jetWithFlavour.first.p4(), math::XYZPoint (0,0,0), flavour);