docs: improve multiset_delta() docs

hydroflow_lang/src/graph/ops/multiset_delta.rs

@@ -5,30 +5,43 @@ use super::{
     WriteContextArgs, RANGE_0, RANGE_1,
 };
 
-// TODO(mingwei): more doc
-/// Multiset delta from the previous tick.
+/// The multiset inverse of [`persist()`](#persist).
+///
+/// > 1 input stream of `T`, 1 output stream of `T`, where `T: Eq + Hash`
+///
+/// For set semantics, [`unique()`](#unique) can be thought of as a "delta" operator, the inverse
+/// of [`persist()`](#persist). In `persist`, new items come in, and all items are repeatedly
+/// released out. Conversely, `unique` take repeated items in, and only releases the new ones out.
+///
+/// This operator does a similar inversion but for multiset semantics, with some caveats. When it
+/// receives duplicate items, instead of ignoring them, it "subtracts" them from the items received
+/// in the previous tick, and only releases new items if there are more than in the previous tick.
+/// However unlike `unique`, this count is only maintained for the previous tick, not over all time.
+///
+/// In the example below, in the second tick two 'a's are removed because two 'a's were received in
+/// the previous tick. The third 'a' is released though.
 ///
 /// ```rustbook
-/// let (input_send, input_recv) = hydroflow::util::unbounded_channel::<u32>();
+/// let (input_send, input_recv) = hydroflow::util::unbounded_channel::<char>();
 /// let mut flow = hydroflow::hydroflow_syntax! {
 ///     source_stream(input_recv)
 ///         -> multiset_delta()
 ///         -> for_each(|n| println!("{}", n));
 /// };
 ///
-/// input_send.send(3).unwrap();
-/// input_send.send(4).unwrap();
-/// input_send.send(3).unwrap();
+/// input_send.send('a').unwrap();
+/// input_send.send('b').unwrap();
+/// input_send.send('a').unwrap();
 /// flow.run_tick();
-/// // 3, 4, 3
+/// // 'a', 'b', 'a'
 ///
-/// input_send.send(3).unwrap();
-/// input_send.send(5).unwrap();
-/// input_send.send(3).unwrap();
-/// input_send.send(3).unwrap();
+/// input_send.send('a').unwrap();
+/// input_send.send('c').unwrap();
+/// input_send.send('a').unwrap();
+/// input_send.send('a').unwrap();
 /// flow.run_tick();
-/// // 5, 3
-/// // First two "3"s are removed due to previous tick.
+/// // 'c', 'a'
+/// // First two 'a's are removed due to previous tick.
 /// ```
 pub const MULTISET_DELTA: OperatorConstraints = OperatorConstraints {
     name: "multiset_delta",

hydroflow_lang/src/graph/ops/persist.rs

@@ -5,7 +5,9 @@ use super::{
     WriteContextArgs, RANGE_0, RANGE_1,
 };
 
-/// Stores each item as it passes through, and replays all item every tick.
+/// Stores each item as it passes through, and replays all items every tick.
+///
+/// > 1 input stream, 1 output stream
 ///
 /// ```hydroflow
 /// // Normally `source_iter(...)` only emits once, but `persist()` will replay the `"hello"`