[claytonwramsey]

Another possible solution for this use case: let each capacity graph store a “dummy” `HashMap<N,(u128, u128)>` for the case where the key is missing. Then, return an iterator over the empty dummy to get an iterator of the same type.

I will however note that simply using an option in the return value is probably the best choice both API- and performance-wise. The consumer may want to do different things if the key doesn’t exist versus if the key has no neighbors. Additionally, returning an Option means there’s only one check for whether the retrieval was correct, rather than checking at every call to `next`.