ssr_core/task/level.rs
use serde::{Deserialize, Serialize};
use std::time::SystemTime;
pub trait TaskLevel<'a>: Default + Serialize + Deserialize<'a> {
type Context;
type SharedState: Default + Serialize + Deserialize<'a>;
fn update(&mut self, shared_state: &mut Self::SharedState, context: Self::Context);
fn next_repetition(&self, _retrievability_goal: f64) -> SystemTime;
}