Taylor R. Brown

perhaps use `std::optional`?

they would also help group different particle filtering algorithms into one base class. For example, if you use [this](https://stackoverflow.com/questions/70920935/is-there-a-semi-pure-virtual-function-in-c/70924135#70924135) as a guide, derived model-specific classes could define models' functions that...

FWIW, I was brought here by the same error message, and for me, it wasn't really a bug. Personally, the reason I was getting it was because I was simulating...