@lnicola,

there are a few spots where I can't see to remove the Box, why is that ?

For example, this does not compile

Hum.. this raises severe problems

Yeah, that won't work. The reason is that impl Iterator is always a single type, but the code in the screenshot would return 5 different types. TBH, maybe Box<dyn Iterator> is not a a terrible choice, as some people don't really like impl Trait return types anyway. I was just pointing out it's an option there.

The reason is that impl Iterator is always a single type, but the code in the screenshot would return 5 different types

the returned type is the same, in the sense they're all Iterators over an Epoch, but the iterator source is different that is true

The closures are also different types, even if they have the same arguments and return value (and I'm not sure these ones do).