Function previous_step 

pub fn previous_step<T>(previous: Previous<T>, current: T) -> Option<T>
where
    T: Clone + PartialEq + Debug + 'static,

Records current against the supplied tracker and returns the snapshot of what was previously recorded.

Convenience wrapper used by component-level consumers that want the “compute previous” + “record new current” steps glued together. Returns None on the first call (no prior value exists yet).

Arguments

• Previous<T> - The tracker obtained from use_previous().
• T - The current value to record.

Returns

• Option<T> - The value that was recorded on the previous call, or None if no prior value exists.