diff --git a/src/collections/inner_group.rs b/src/collections/inner_group.rs
index 554b66b..0434068 100644
--- a/src/collections/inner_group.rs
+++ b/src/collections/inner_group.rs
@@ -1,4 +1,5 @@
 use core::{
+    future::Future,
     marker::PhantomData,
     ops::ControlFlow,
     pin::Pin,
@@ -6,6 +7,7 @@ use core::{
 };
 
 use alloc::{collections::BTreeSet, fmt};
+use futures_core::Stream;
 use slab::Slab;
 use smallvec::{smallvec, SmallVec};
 
@@ -146,19 +148,14 @@ impl<A, B> fmt::Debug for InnerGroup<A, B> {
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct Key(pub usize);
 
-// ----
-// prototyping
-
-use theory::*;
-
 impl<A, B> InnerGroup<A, B>
 where
-    B: PollBehavior<Polling = A>,
+    B: PollBehavior<A>,
 {
     pub fn poll_next_inner(
         mut self: Pin<&mut Self>,
         cx: &Context<'_>,
-    ) -> Poll<Option<(Key, B::Poll)>> {
+    ) -> Poll<Option<(Key, B::Output)>> {
         let this = unsafe { self.as_mut().get_unchecked_mut() };
         // short-circuit if we have no items to iterate over
         if this.is_empty() {
@@ -189,26 +186,26 @@ where
 
             let pollable = unsafe { Pin::new_unchecked(&mut this.items[index]) };
             match B::poll(pollable, &mut cx) {
-                Poll::Ready(ControlFlow::Break((result, PollAgain::Stop))) => {
+                ControlFlow::Break((result, PollAgain::Stop)) => {
                     for item in removal_queue {
                         this.remove(Key(item));
                     }
                     this.remove(Key(index));
                     return Poll::Ready(Some((Key(index), result)));
                 }
-                Poll::Ready(ControlFlow::Break((result, PollAgain::Poll))) => {
+                ControlFlow::Break((result, PollAgain::Poll)) => {
                     this.states[index].set_pending();
                     this.wakers.readiness().set_ready(index);
 
                     ret = Poll::Ready(Some((Key(index), result)));
                     break;
                 }
-                Poll::Ready(ControlFlow::Continue(_)) => {
+                ControlFlow::Continue(PollAgain::Stop) => {
                     done_count += 1;
                     removal_queue.push(index);
                     continue;
                 }
-                Poll::Pending => continue,
+                ControlFlow::Continue(PollAgain::Poll) => continue,
             }
         }
         for item in removal_queue {
@@ -223,68 +220,74 @@ where
     }
 }
 
-pub mod theory {
-    use core::future::Future;
-    use core::marker::PhantomData;
-    use core::ops::ControlFlow;
-    use core::pin::Pin;
-    use core::task::{Context, Poll};
-
-    use futures_core::Stream;
-
-    pub enum PollAgain {
-        Poll,
-        Stop,
-    }
-
-    pub trait PollBehavior {
-        type Item;
-        type Polling;
-        type Poll;
+/// Used to tell the callee of [`PollBehavior::poll`]
+/// whether the `poll`'ed item should be polled again.
+pub(crate) enum PollAgain {
+    /// Keep polling
+    Poll,
+    /// Stop polling
+    Stop,
+}
 
-        fn poll(
-            this: Pin<&mut Self::Polling>,
-            cx: &mut Context<'_>,
-        ) -> Poll<ControlFlow<(Self::Poll, PollAgain)>>;
-    }
+pub(crate) trait PollBehavior<P> {
+    /// The output type of the polled item
+    type Output;
+
+    /// Poll the underlying item and decides how the iteration should proceed
+    ///
+    /// # Return value
+    /// The returned value coordinates two key aspects of the group iteration:
+    /// - whether the group should keep iterating over the next items;
+    ///    - `ControlFlow::Continue(_)` to inform that the group
+    ///       should proceed to the next item
+    ///    - `ControlFlow::Break(_)` to inform that the group
+    ///       should stop iterating
+    /// - whether the group should poll the same item again;
+    ///    - [`PollAgain::Poll`] to inform the group to
+    ///      mark that the item should be polled again
+    ///    - [`PollAgain::Stop`] to inform the group to
+    ///      stop polling this item
+    fn poll(
+        this: Pin<&mut P>,
+        cx: &mut Context<'_>,
+    ) -> ControlFlow<(Self::Output, PollAgain), PollAgain>;
+}
 
-    #[derive(Debug)]
-    pub struct PollFuture<F>(PhantomData<F>);
-
-    impl<F: Future> PollBehavior for PollFuture<F> {
-        type Item = F::Output;
-        type Polling = F;
-        type Poll = Self::Item;
-
-        fn poll(
-            this: Pin<&mut Self::Polling>,
-            cx: &mut Context<'_>,
-        ) -> Poll<ControlFlow<(Self::Poll, PollAgain)>> {
-            if let Poll::Ready(item) = this.poll(cx) {
-                Poll::Ready(ControlFlow::Break((item, PollAgain::Stop)))
-            } else {
-                Poll::Pending
-            }
+pub struct PollFuture;
+
+impl<F: Future> PollBehavior<F> for PollFuture {
+    type Output = F::Output;
+
+    fn poll(
+        future: Pin<&mut F>,
+        cx: &mut Context<'_>,
+    ) -> ControlFlow<(Self::Output, PollAgain), PollAgain> {
+        if let Poll::Ready(output) = future.poll(cx) {
+            // return the futures output and inform the group to not poll it again
+            ControlFlow::Break((output, PollAgain::Stop))
+        } else {
+            // future is not ready yet, keep polling
+            ControlFlow::Continue(PollAgain::Poll)
         }
     }
+}
 
-    #[derive(Debug)]
-    pub struct PollStream<S>(PhantomData<S>);
-
-    impl<S: Stream> PollBehavior for PollStream<S> {
-        type Item = S::Item;
-        type Polling = S;
-        type Poll = Self::Item;
-
-        fn poll(
-            this: Pin<&mut Self::Polling>,
-            cx: &mut Context<'_>,
-        ) -> Poll<ControlFlow<(Self::Poll, PollAgain)>> {
-            match this.poll_next(cx) {
-                Poll::Ready(Some(item)) => Poll::Ready(ControlFlow::Break((item, PollAgain::Poll))),
-                Poll::Ready(None) => Poll::Ready(ControlFlow::Continue(())),
-                Poll::Pending => Poll::Pending,
-            }
+pub struct PollStream;
+
+impl<S: Stream> PollBehavior<S> for PollStream {
+    type Output = S::Item;
+
+    fn poll(
+        stream: Pin<&mut S>,
+        cx: &mut Context<'_>,
+    ) -> ControlFlow<(Self::Output, PollAgain), PollAgain> {
+        match stream.poll_next(cx) {
+            // stop the iteration, keep polling this stream
+            Poll::Ready(Some(item)) => ControlFlow::Break((item, PollAgain::Poll)),
+            // continue the iteration, stop polling this stream
+            Poll::Ready(None) => ControlFlow::Continue(PollAgain::Stop),
+            // continue the iteration, continue polling this stream
+            Poll::Pending => ControlFlow::Continue(PollAgain::Poll),
         }
     }
 }
diff --git a/src/future/future_group.rs b/src/future/future_group.rs
index 27955a8..23fdb66 100644
--- a/src/future/future_group.rs
+++ b/src/future/future_group.rs
@@ -4,8 +4,7 @@ use core::task::{Context, Poll};
 use futures_core::stream::Stream;
 use futures_core::Future;
 
-use crate::collections::inner_group::theory::PollFuture;
-use crate::collections::inner_group::{InnerGroup, Key};
+use crate::collections::inner_group::{InnerGroup, Key, PollFuture};
 
 /// A growable group of futures which act as a single unit.
 ///
@@ -60,7 +59,7 @@ use crate::collections::inner_group::{InnerGroup, Key};
 #[pin_project::pin_project]
 pub struct FutureGroup<F> {
     #[pin]
-    inner: InnerGroup<F, PollFuture<F>>,
+    inner: InnerGroup<F, PollFuture>,
 }
 
 impl<F> FutureGroup<F> {
diff --git a/src/stream/stream_group.rs b/src/stream/stream_group.rs
index b6d4773..364c70a 100644
--- a/src/stream/stream_group.rs
+++ b/src/stream/stream_group.rs
@@ -4,8 +4,7 @@ use core::pin::Pin;
 use core::task::{Context, Poll};
 use futures_core::Stream;
 
-use crate::collections::inner_group::theory::PollStream;
-use crate::collections::inner_group::{InnerGroup, Key};
+use crate::collections::inner_group::{InnerGroup, Key, PollStream};
 
 /// A growable group of streams which act as a single unit.
 ///
@@ -59,7 +58,7 @@ use crate::collections::inner_group::{InnerGroup, Key};
 #[pin_project::pin_project]
 pub struct StreamGroup<S> {
     #[pin]
-    inner: InnerGroup<S, PollStream<S>>,
+    inner: InnerGroup<S, PollStream>,
 }
 
 impl<S> StreamGroup<S> {