val next_step : 'a Base.Sequence.t -> ('a, 'a Base.Sequence.t) Base.Sequence.Step.tnext_step returns the next step in a sequence's construction. It is like next, but it also allows observing Skip steps.
val delayed_fold_step : 'a Base.Sequence.t -> init:'s -> f:('s -> 'a option -> k:('s -> 'r) -> 'r) -> finish:('s -> 'r) -> 'rdelayed_fold_step is liked delayed_fold, but f takes an option where None represents a Skip step.