[shadaj]

Flo lead-author here! This is spot on :) Flo aims to be a bit less opinionated than Timely in how the runtime should behave, so in particular we don't support the type of "time-traveling" computation that Timely needs when you have iterative computations on datasets with retractions.

This is also one of the core differences of Timely compared to DBSP, which uses a flat representation (z-sets) to store retractions rather than using versioned elements. This allows retractions to be propagated as just negative item counts which fits into the Flo model (and therefore Hydro).