## Differential dataflow to-do list: 1. Batch builders need an "ordered" option where the keys and vals are already sorted (cf group.output). 2. Several trace implementations need to be fleshed out (e.g. `time` and `constant`). - Consider ConstantCollection type which can only be construct from known data, arranged to `constant`. 3. Several trace implementations could benefit from a RHH `keys` field; prototype and test! - Probably wants a Uniform<T: Unsigned> struct for "node identifiers"; needs tweaks to `Data` trait. 4. Lots of sorting, but no radix-sorting. Historically a big improvement. - Connects to U: Unsigned output of `hashed`; no point radix-sorting u32 keys as if u64s. 5. Several operators need revision: distinct, threshold, cogroup. 6. The `keys` trace implementation has had zero testing. Important! 7. Progressive merging under-explored; trade-offs in rate of work? (yes, but worth?) 8. High-resolution times aren't too far away. - Think up alternate Collection type with new data bits. - Uncomment `group` implementation and get to work. 9. Join now has "deferred work"; check it out to see if it helps on large graphs.