A left fold over events in feed order; every view of a trajectory is one of these, so state after n + k events
is step applied k times to the state after n, and a reader never has to start from the first event.
Folds a batch at once, equal to stepping each event; lets a reducer copy its state once per batch rather than once
per event, which keeps a long fold linear. The state passed in is never modified.
A left fold over events in feed order; every view of a trajectory is one of these, so state after
n + kevents isstepappliedktimes to the state aftern, and a reader never has to start from the first event.