From fddeaa0af740ac4176341ba0019ccc696a3c96dd Mon Sep 17 00:00:00 2001
From: Hanno Braun <hanno@braun-odw.eu>
Date: Thu, 21 Sep 2023 09:03:26 +0200
Subject: [PATCH 01/14] Remove unnecessary lifetime bounds

---
 crates/fj-core/src/objects/kinds/region.rs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/crates/fj-core/src/objects/kinds/region.rs b/crates/fj-core/src/objects/kinds/region.rs
index f39b8072f..b861db472 100644
--- a/crates/fj-core/src/objects/kinds/region.rs
+++ b/crates/fj-core/src/objects/kinds/region.rs
@@ -40,12 +40,12 @@ impl Region {
     /// Access the cycles that bound the region on the inside
     ///
     /// Each of these cycles defines a hole in the region .
-    pub fn interiors(&self) -> impl Iterator<Item = &Handle<Cycle>> + '_ {
+    pub fn interiors(&self) -> impl Iterator<Item = &Handle<Cycle>> {
         self.interiors.iter()
     }
 
     /// Access all cycles of the region (both exterior and interior)
-    pub fn all_cycles(&self) -> impl Iterator<Item = &Handle<Cycle>> + '_ {
+    pub fn all_cycles(&self) -> impl Iterator<Item = &Handle<Cycle>> {
         [self.exterior()].into_iter().chain(self.interiors())
     }
 

From 6c54c2f4fe7ae5ad243c086c9eda5249ec2cb8cd Mon Sep 17 00:00:00 2001
From: Hanno Braun <hanno@braun-odw.eu>
Date: Thu, 21 Sep 2023 09:30:56 +0200
Subject: [PATCH 02/14] Remove redundant type hint

---
 crates/fj-core/src/objects/kinds/cycle.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/crates/fj-core/src/objects/kinds/cycle.rs b/crates/fj-core/src/objects/kinds/cycle.rs
index 0af5f5b87..0c6af8a90 100644
--- a/crates/fj-core/src/objects/kinds/cycle.rs
+++ b/crates/fj-core/src/objects/kinds/cycle.rs
@@ -12,7 +12,7 @@ pub struct Cycle {
 impl Cycle {
     /// Create an instance of `Cycle`
     pub fn new(edges: impl IntoIterator<Item = Handle<Edge>>) -> Self {
-        let edges = edges.into_iter().collect::<Vec<_>>();
+        let edges = edges.into_iter().collect();
         Self { edges }
     }
 

From 2575af05f18919c17e898d8640f3c0388b102493 Mon Sep 17 00:00:00 2001
From: Hanno Braun <hanno@braun-odw.eu>
Date: Thu, 21 Sep 2023 10:22:15 +0200
Subject: [PATCH 03/14] Add `HandleSet`

---
 crates/fj-core/src/objects/handles.rs | 69 +++++++++++++++++++++++++++
 crates/fj-core/src/objects/mod.rs     |  1 +
 2 files changed, 70 insertions(+)
 create mode 100644 crates/fj-core/src/objects/handles.rs

diff --git a/crates/fj-core/src/objects/handles.rs b/crates/fj-core/src/objects/handles.rs
new file mode 100644
index 000000000..777d5eeeb
--- /dev/null
+++ b/crates/fj-core/src/objects/handles.rs
@@ -0,0 +1,69 @@
+use std::{collections::BTreeSet, fmt::Debug};
+
+use crate::storage::Handle;
+
+/// An ordered set of object handles
+///
+/// This is an internal data structure that is used within objects that
+/// reference multiple other objects of the same type. It does not contain any
+/// duplicate elements, and it maintains the insertion order of those elements.
+///
+/// `HandleSet` implement `FromIterator`, but it must never be constructed from
+/// an iterator that contains duplicate handles. This will result in a panic.
+#[derive(Clone, Debug, Eq, PartialEq, Hash, Ord, PartialOrd)]
+pub struct HandleSet<T> {
+    // This is supposed to be a set data structure, so what is that `Vec` doing
+    // here? Well, it's here because we need it to preserve insertion order, but
+    // that doesn't explain why it is here *alone*.
+    //
+    // If you look closely, you'll notice that this is an immutable data
+    // structure (since it is used in objects, and objects themselves are
+    // immutable). We make sure there are no duplicates when this is
+    // constructed (see the `FromIterator` implementation below), but after
+    // that, we're fine.
+    inner: Vec<Handle<T>>,
+}
+
+impl<T> HandleSet<T> {
+    pub fn len(&self) -> usize {
+        self.inner.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.inner.is_empty()
+    }
+
+    pub fn nth(&self, index: usize) -> Option<&Handle<T>> {
+        self.inner.get(index)
+    }
+
+    pub fn iter(
+        &self,
+    ) -> impl Iterator<Item = &Handle<T>> + Clone + ExactSizeIterator {
+        self.inner.iter()
+    }
+}
+
+impl<O> FromIterator<Handle<O>> for HandleSet<O>
+where
+    O: Debug + Ord,
+{
+    fn from_iter<T: IntoIterator<Item = Handle<O>>>(handles: T) -> Self {
+        let mut added = BTreeSet::new();
+        let mut inner = Vec::new();
+
+        for handle in handles {
+            if added.contains(&handle) {
+                panic!(
+                    "Constructing `HandleSet` with duplicate handle: {:?}",
+                    handle
+                );
+            }
+
+            added.insert(handle.clone());
+            inner.push(handle);
+        }
+
+        Self { inner }
+    }
+}
diff --git a/crates/fj-core/src/objects/mod.rs b/crates/fj-core/src/objects/mod.rs
index 8beedc65b..29c54e388 100644
--- a/crates/fj-core/src/objects/mod.rs
+++ b/crates/fj-core/src/objects/mod.rs
@@ -39,6 +39,7 @@
 //!
 //! [`Handle`]: crate::storage::Handle
 
+mod handles;
 mod kinds;
 mod object;
 mod set;

From 05b00effd70b146489a6e339283563806f1ce9a9 Mon Sep 17 00:00:00 2001
From: Hanno Braun <hanno@braun-odw.eu>
Date: Thu, 21 Sep 2023 10:24:03 +0200
Subject: [PATCH 04/14] Use `HandleSet` in `Cycle`

---
 crates/fj-core/src/objects/kinds/cycle.rs | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/crates/fj-core/src/objects/kinds/cycle.rs b/crates/fj-core/src/objects/kinds/cycle.rs
index 0c6af8a90..8a5389e34 100644
--- a/crates/fj-core/src/objects/kinds/cycle.rs
+++ b/crates/fj-core/src/objects/kinds/cycle.rs
@@ -1,12 +1,16 @@
 use fj_math::{Scalar, Winding};
 use itertools::Itertools;
 
-use crate::{geometry::SurfacePath, objects::Edge, storage::Handle};
+use crate::{
+    geometry::SurfacePath,
+    objects::{handles::HandleSet, Edge},
+    storage::Handle,
+};
 
 /// A cycle of connected edges
 #[derive(Clone, Debug, Eq, PartialEq, Hash, Ord, PartialOrd)]
 pub struct Cycle {
-    edges: Vec<Handle<Edge>>,
+    edges: HandleSet<Edge>,
 }
 
 impl Cycle {
@@ -30,7 +34,7 @@ impl Cycle {
 
     /// Access the edge with the provided index
     pub fn nth_edge(&self, index: usize) -> Option<&Handle<Edge>> {
-        self.edges.get(index)
+        self.edges.nth(index)
     }
 
     /// Access the edge after the provided one
@@ -39,7 +43,7 @@ impl Cycle {
     pub fn edge_after(&self, edge: &Handle<Edge>) -> Option<&Handle<Edge>> {
         self.index_of(edge).map(|index| {
             let next_index = (index + 1) % self.edges.len();
-            &self.edges[next_index]
+            self.edges.nth(next_index).unwrap()
         })
     }
 

From fec81cf2c71b7ae3a7ef604e90967422cc3ea40c Mon Sep 17 00:00:00 2001
From: Hanno Braun <hanno@braun-odw.eu>
Date: Thu, 21 Sep 2023 10:25:09 +0200
Subject: [PATCH 05/14] Use `HandleSet` in `Region`, `Sketch`, `Solid`

---
 crates/fj-core/src/objects/kinds/region.rs | 7 +++++--
 crates/fj-core/src/objects/kinds/sketch.rs | 6 ++----
 crates/fj-core/src/objects/kinds/solid.rs  | 9 +++++----
 3 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/crates/fj-core/src/objects/kinds/region.rs b/crates/fj-core/src/objects/kinds/region.rs
index b861db472..40c0d3336 100644
--- a/crates/fj-core/src/objects/kinds/region.rs
+++ b/crates/fj-core/src/objects/kinds/region.rs
@@ -1,7 +1,10 @@
 //! A single, continues 2d region
 use fj_interop::mesh::Color;
 
-use crate::{objects::Cycle, storage::Handle};
+use crate::{
+    objects::{handles::HandleSet, Cycle},
+    storage::Handle,
+};
 
 /// A single, continuous 2d region, may contain holes
 ///
@@ -14,7 +17,7 @@ use crate::{objects::Cycle, storage::Handle};
 #[derive(Clone, Debug, Eq, PartialEq, Hash, Ord, PartialOrd)]
 pub struct Region {
     exterior: Handle<Cycle>,
-    interiors: Vec<Handle<Cycle>>,
+    interiors: HandleSet<Cycle>,
     color: Option<Color>,
 }
 
diff --git a/crates/fj-core/src/objects/kinds/sketch.rs b/crates/fj-core/src/objects/kinds/sketch.rs
index 7cce6806e..163f11e7e 100644
--- a/crates/fj-core/src/objects/kinds/sketch.rs
+++ b/crates/fj-core/src/objects/kinds/sketch.rs
@@ -1,7 +1,5 @@
-use std::collections::BTreeSet;
-
 use crate::{
-    objects::{Face, FaceSet, Region, Surface},
+    objects::{handles::HandleSet, Face, FaceSet, Region, Surface},
     operations::Insert,
     services::Services,
     storage::Handle,
@@ -10,7 +8,7 @@ use crate::{
 /// A 2-dimensional shape
 #[derive(Clone, Debug, Eq, PartialEq, Hash, Ord, PartialOrd)]
 pub struct Sketch {
-    regions: BTreeSet<Handle<Region>>,
+    regions: HandleSet<Region>,
 }
 
 impl Sketch {
diff --git a/crates/fj-core/src/objects/kinds/solid.rs b/crates/fj-core/src/objects/kinds/solid.rs
index 24aba5cdb..fefd1b32b 100644
--- a/crates/fj-core/src/objects/kinds/solid.rs
+++ b/crates/fj-core/src/objects/kinds/solid.rs
@@ -1,6 +1,7 @@
-use std::collections::BTreeSet;
-
-use crate::{objects::Shell, storage::Handle};
+use crate::{
+    objects::{handles::HandleSet, Shell},
+    storage::Handle,
+};
 
 /// A 3-dimensional shape, built from [`Shell`]s. Many Solids will contains only
 /// one shell, but if the Solid contains cavities they will be represented by a
@@ -12,7 +13,7 @@ use crate::{objects::Shell, storage::Handle};
 /// not currently validated.
 #[derive(Clone, Debug, Eq, PartialEq, Hash, Ord, PartialOrd)]
 pub struct Solid {
-    shells: BTreeSet<Handle<Shell>>,
+    shells: HandleSet<Shell>,
 }
 
 impl Solid {

From 9cf1bcbbf72ab1718380a2332d369cc51a191d9f Mon Sep 17 00:00:00 2001
From: Hanno Braun <hanno@braun-odw.eu>
Date: Thu, 21 Sep 2023 10:56:40 +0200
Subject: [PATCH 06/14] Add `HandleIter`

---
 crates/fj-core/src/objects/handles.rs | 54 +++++++++++++++++++++++++--
 crates/fj-core/src/objects/mod.rs     |  1 +
 2 files changed, 51 insertions(+), 4 deletions(-)

diff --git a/crates/fj-core/src/objects/handles.rs b/crates/fj-core/src/objects/handles.rs
index 777d5eeeb..518729e6b 100644
--- a/crates/fj-core/src/objects/handles.rs
+++ b/crates/fj-core/src/objects/handles.rs
@@ -37,10 +37,11 @@ impl<T> HandleSet<T> {
         self.inner.get(index)
     }
 
-    pub fn iter(
-        &self,
-    ) -> impl Iterator<Item = &Handle<T>> + Clone + ExactSizeIterator {
-        self.inner.iter()
+    pub fn iter(&self) -> HandleIter<T> {
+        HandleIter {
+            handles: &self.inner,
+            next_index: 0,
+        }
     }
 }
 
@@ -67,3 +68,48 @@ where
         Self { inner }
     }
 }
+
+/// An iterator over handles to objects
+///
+/// This struct is returned by the respective methods of all objects that
+/// reference multiple objects of the same type.
+pub struct HandleIter<'r, T> {
+    handles: &'r Vec<Handle<T>>,
+    next_index: usize,
+}
+
+impl<'r, T> Iterator for HandleIter<'r, T> {
+    // You might wonder why we're returning references to handles here, when
+    // `Handle` already is kind of reference, and easily cloned.
+    //
+    // Most of the time that doesn't make a difference, but there are use cases
+    // where dealing with owned `Handle`s is inconvenient, for example when
+    // using iterator adapters. You can't return a reference to the argument of
+    // an adapter's closure, if you own that argument. You can, if you just
+    // reference the argument.
+    type Item = &'r Handle<T>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let handle = self.handles.get(self.next_index);
+        self.next_index += 1;
+        handle
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let size = self.handles.len();
+        (size, Some(size))
+    }
+}
+
+impl<T> ExactSizeIterator for HandleIter<'_, T> {}
+
+// Deriving won't work, as that only derives `Clone` where `T: Clone`. But
+// `HandleIter` can be `Clone`d unconditionally.
+impl<T> Clone for HandleIter<'_, T> {
+    fn clone(&self) -> Self {
+        Self {
+            handles: self.handles,
+            next_index: self.next_index,
+        }
+    }
+}
diff --git a/crates/fj-core/src/objects/mod.rs b/crates/fj-core/src/objects/mod.rs
index 29c54e388..c69fb8707 100644
--- a/crates/fj-core/src/objects/mod.rs
+++ b/crates/fj-core/src/objects/mod.rs
@@ -46,6 +46,7 @@ mod set;
 mod stores;
 
 pub use self::{
+    handles::HandleIter,
     kinds::{
         curve::Curve,
         cycle::Cycle,

From d53f3d4abedaaf09ba175c5b2a74abae307a7275 Mon Sep 17 00:00:00 2001
From: Hanno Braun <hanno@braun-odw.eu>
Date: Thu, 21 Sep 2023 11:00:21 +0200
Subject: [PATCH 07/14] Return `HandleIter`, where possible

---
 crates/fj-core/src/objects/kinds/cycle.rs  | 4 ++--
 crates/fj-core/src/objects/kinds/region.rs | 4 ++--
 crates/fj-core/src/objects/kinds/sketch.rs | 4 ++--
 crates/fj-core/src/objects/kinds/solid.rs  | 4 ++--
 4 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/crates/fj-core/src/objects/kinds/cycle.rs b/crates/fj-core/src/objects/kinds/cycle.rs
index 8a5389e34..12334f9b5 100644
--- a/crates/fj-core/src/objects/kinds/cycle.rs
+++ b/crates/fj-core/src/objects/kinds/cycle.rs
@@ -3,7 +3,7 @@ use itertools::Itertools;
 
 use crate::{
     geometry::SurfacePath,
-    objects::{handles::HandleSet, Edge},
+    objects::{handles::HandleSet, Edge, HandleIter},
     storage::Handle,
 };
 
@@ -21,7 +21,7 @@ impl Cycle {
     }
 
     /// Access the edges that make up the cycle
-    pub fn edges(&self) -> impl Iterator<Item = &Handle<Edge>> {
+    pub fn edges(&self) -> HandleIter<Edge> {
         self.edges.iter()
     }
 
diff --git a/crates/fj-core/src/objects/kinds/region.rs b/crates/fj-core/src/objects/kinds/region.rs
index 40c0d3336..28ea8e5eb 100644
--- a/crates/fj-core/src/objects/kinds/region.rs
+++ b/crates/fj-core/src/objects/kinds/region.rs
@@ -2,7 +2,7 @@
 use fj_interop::mesh::Color;
 
 use crate::{
-    objects::{handles::HandleSet, Cycle},
+    objects::{handles::HandleSet, Cycle, HandleIter},
     storage::Handle,
 };
 
@@ -43,7 +43,7 @@ impl Region {
     /// Access the cycles that bound the region on the inside
     ///
     /// Each of these cycles defines a hole in the region .
-    pub fn interiors(&self) -> impl Iterator<Item = &Handle<Cycle>> {
+    pub fn interiors(&self) -> HandleIter<Cycle> {
         self.interiors.iter()
     }
 
diff --git a/crates/fj-core/src/objects/kinds/sketch.rs b/crates/fj-core/src/objects/kinds/sketch.rs
index 163f11e7e..657eb1270 100644
--- a/crates/fj-core/src/objects/kinds/sketch.rs
+++ b/crates/fj-core/src/objects/kinds/sketch.rs
@@ -1,5 +1,5 @@
 use crate::{
-    objects::{handles::HandleSet, Face, FaceSet, Region, Surface},
+    objects::{handles::HandleSet, Face, FaceSet, HandleIter, Region, Surface},
     operations::Insert,
     services::Services,
     storage::Handle,
@@ -20,7 +20,7 @@ impl Sketch {
     }
 
     /// Access the regions of the sketch
-    pub fn regions(&self) -> impl Iterator<Item = &Handle<Region>> {
+    pub fn regions(&self) -> HandleIter<Region> {
         self.regions.iter()
     }
 
diff --git a/crates/fj-core/src/objects/kinds/solid.rs b/crates/fj-core/src/objects/kinds/solid.rs
index fefd1b32b..fe973df9d 100644
--- a/crates/fj-core/src/objects/kinds/solid.rs
+++ b/crates/fj-core/src/objects/kinds/solid.rs
@@ -1,5 +1,5 @@
 use crate::{
-    objects::{handles::HandleSet, Shell},
+    objects::{handles::HandleSet, HandleIter, Shell},
     storage::Handle,
 };
 
@@ -25,7 +25,7 @@ impl Solid {
     }
 
     /// Access the solid's shells
-    pub fn shells(&self) -> impl Iterator<Item = &Handle<Shell>> {
+    pub fn shells(&self) -> HandleIter<Shell> {
         self.shells.iter()
     }
 }

From 8634c581c8a85fd5b4b32c7db4730bd491062cc4 Mon Sep 17 00:00:00 2001
From: Hanno Braun <hanno@braun-odw.eu>
Date: Thu, 21 Sep 2023 11:03:30 +0200
Subject: [PATCH 08/14] Add comment

---
 crates/fj-core/src/objects/kinds/region.rs | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/crates/fj-core/src/objects/kinds/region.rs b/crates/fj-core/src/objects/kinds/region.rs
index 28ea8e5eb..e9028b5e2 100644
--- a/crates/fj-core/src/objects/kinds/region.rs
+++ b/crates/fj-core/src/objects/kinds/region.rs
@@ -49,6 +49,11 @@ impl Region {
 
     /// Access all cycles of the region (both exterior and interior)
     pub fn all_cycles(&self) -> impl Iterator<Item = &Handle<Cycle>> {
+        // It would be nice to return `HandleIter` here, but there's no
+        // straight-forward way to chain it to another iterator, given it's
+        // current implementation.
+        //
+        // Maybe this can be addressed, once the need arises.
         [self.exterior()].into_iter().chain(self.interiors())
     }
 

From 77501c33e2f82053b88f888191acdae68bbdcb93 Mon Sep 17 00:00:00 2001
From: Hanno Braun <hanno@braun-odw.eu>
Date: Thu, 21 Sep 2023 11:08:02 +0200
Subject: [PATCH 09/14] Replace `FaceSet` in `Shell`

---
 crates/fj-core/src/algorithms/approx/face.rs     | 4 ++--
 crates/fj-core/src/algorithms/transform/shell.rs | 8 ++++----
 crates/fj-core/src/objects/kinds/shell.rs        | 8 ++++----
 crates/fj-core/src/operations/update/shell.rs    | 5 ++---
 4 files changed, 12 insertions(+), 13 deletions(-)

diff --git a/crates/fj-core/src/algorithms/approx/face.rs b/crates/fj-core/src/algorithms/approx/face.rs
index 6cccce275..3babf79d3 100644
--- a/crates/fj-core/src/algorithms/approx/face.rs
+++ b/crates/fj-core/src/algorithms/approx/face.rs
@@ -7,7 +7,7 @@ use std::{collections::BTreeSet, ops::Deref};
 use fj_interop::mesh::Color;
 
 use crate::{
-    objects::{Face, FaceSet, Handedness},
+    objects::{Face, Handedness, HandleIter},
     validate::ValidationConfig,
 };
 
@@ -15,7 +15,7 @@ use super::{
     cycle::CycleApprox, edge::EdgeApproxCache, Approx, ApproxPoint, Tolerance,
 };
 
-impl Approx for &FaceSet {
+impl Approx for HandleIter<'_, Face> {
     type Approximation = BTreeSet<FaceApprox>;
     type Cache = EdgeApproxCache;
 
diff --git a/crates/fj-core/src/algorithms/transform/shell.rs b/crates/fj-core/src/algorithms/transform/shell.rs
index a8d91e0f9..0d1d474e0 100644
--- a/crates/fj-core/src/algorithms/transform/shell.rs
+++ b/crates/fj-core/src/algorithms/transform/shell.rs
@@ -11,10 +11,10 @@ impl TransformObject for Shell {
         services: &mut Services,
         cache: &mut TransformCache,
     ) -> Self {
-        let faces =
-            self.faces().clone().into_iter().map(|face| {
-                face.transform_with_cache(transform, services, cache)
-            });
+        let faces = self
+            .faces()
+            .cloned()
+            .map(|face| face.transform_with_cache(transform, services, cache));
 
         Self::new(faces)
     }
diff --git a/crates/fj-core/src/objects/kinds/shell.rs b/crates/fj-core/src/objects/kinds/shell.rs
index 8a1717255..c8654945f 100644
--- a/crates/fj-core/src/objects/kinds/shell.rs
+++ b/crates/fj-core/src/objects/kinds/shell.rs
@@ -1,12 +1,12 @@
 use crate::{
-    objects::{Face, FaceSet},
+    objects::{handles::HandleSet, Face, HandleIter},
     storage::Handle,
 };
 
 /// A 3-dimensional closed shell
 #[derive(Clone, Debug, Eq, PartialEq, Hash, Ord, PartialOrd)]
 pub struct Shell {
-    faces: FaceSet,
+    faces: HandleSet<Face>,
 }
 
 impl Shell {
@@ -18,7 +18,7 @@ impl Shell {
     }
 
     /// Access the faces of the shell
-    pub fn faces(&self) -> &FaceSet {
-        &self.faces
+    pub fn faces(&self) -> HandleIter<Face> {
+        self.faces.iter()
     }
 }
diff --git a/crates/fj-core/src/operations/update/shell.rs b/crates/fj-core/src/operations/update/shell.rs
index cd68d70e4..52c9c6f36 100644
--- a/crates/fj-core/src/operations/update/shell.rs
+++ b/crates/fj-core/src/operations/update/shell.rs
@@ -24,7 +24,7 @@ pub trait UpdateShell {
 
 impl UpdateShell for Shell {
     fn add_faces(&self, faces: impl IntoIterator<Item = Handle<Face>>) -> Self {
-        let faces = self.faces().into_iter().cloned().chain(faces);
+        let faces = self.faces().cloned().chain(faces);
         Shell::new(faces)
     }
 
@@ -33,7 +33,7 @@ impl UpdateShell for Shell {
         original: &Handle<Face>,
         replacement: Handle<Face>,
     ) -> Self {
-        let faces = self.faces().into_iter().map(|face| {
+        let faces = self.faces().map(|face| {
             if face.id() == original.id() {
                 replacement.clone()
             } else {
@@ -47,7 +47,6 @@ impl UpdateShell for Shell {
     fn remove_face(&self, handle: &Handle<Face>) -> Self {
         let faces = self
             .faces()
-            .into_iter()
             .filter(|face| face.id() == handle.id())
             .cloned();
 

From ca34d169a63062e86c470041f554a1da59447d46 Mon Sep 17 00:00:00 2001
From: Hanno Braun <hanno@braun-odw.eu>
Date: Thu, 21 Sep 2023 11:09:00 +0200
Subject: [PATCH 10/14] Remove unused trait implementation

---
 .../fj-core/src/algorithms/transform/face.rs  | 19 +------------------
 1 file changed, 1 insertion(+), 18 deletions(-)

diff --git a/crates/fj-core/src/algorithms/transform/face.rs b/crates/fj-core/src/algorithms/transform/face.rs
index 7c99c97dc..1aa6e2278 100644
--- a/crates/fj-core/src/algorithms/transform/face.rs
+++ b/crates/fj-core/src/algorithms/transform/face.rs
@@ -1,7 +1,7 @@
 use fj_math::Transform;
 
 use crate::{
-    objects::{Face, FaceSet, Region},
+    objects::{Face, Region},
     operations::Insert,
     services::Services,
 };
@@ -36,20 +36,3 @@ impl TransformObject for Face {
         Self::new(surface, region)
     }
 }
-
-impl TransformObject for FaceSet {
-    fn transform_with_cache(
-        self,
-        transform: &Transform,
-        services: &mut Services,
-        cache: &mut TransformCache,
-    ) -> Self {
-        let mut faces = Self::new();
-        faces.extend(
-            self.into_iter().map(|face| {
-                face.transform_with_cache(transform, services, cache)
-            }),
-        );
-        faces
-    }
-}

From 1865f3db9bb4a5c9d61d5a12f0a87519aa3c99ef Mon Sep 17 00:00:00 2001
From: Hanno Braun <hanno@braun-odw.eu>
Date: Thu, 21 Sep 2023 11:11:43 +0200
Subject: [PATCH 11/14] Refactor

---
 crates/fj-core/src/objects/kinds/sketch.rs | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/crates/fj-core/src/objects/kinds/sketch.rs b/crates/fj-core/src/objects/kinds/sketch.rs
index 657eb1270..8deb53d9a 100644
--- a/crates/fj-core/src/objects/kinds/sketch.rs
+++ b/crates/fj-core/src/objects/kinds/sketch.rs
@@ -30,8 +30,7 @@ impl Sketch {
         surface: Handle<Surface>,
         services: &mut Services,
     ) -> FaceSet {
-        self.regions
-            .iter()
+        self.regions()
             .map(|region| {
                 Face::new(surface.clone(), region.clone()).insert(services)
             })

From 7e6e5bae9e161366ab3ed4fc516e9aa62131414f Mon Sep 17 00:00:00 2001
From: Hanno Braun <hanno@braun-odw.eu>
Date: Thu, 21 Sep 2023 11:12:45 +0200
Subject: [PATCH 12/14] Refactor to improve clarity

---
 crates/fj-core/src/algorithms/sweep/sketch.rs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/crates/fj-core/src/algorithms/sweep/sketch.rs b/crates/fj-core/src/algorithms/sweep/sketch.rs
index c50b06777..6b95c04b1 100644
--- a/crates/fj-core/src/algorithms/sweep/sketch.rs
+++ b/crates/fj-core/src/algorithms/sweep/sketch.rs
@@ -18,10 +18,11 @@ impl Sweep for (Handle<Sketch>, Handle<Surface>) {
         cache: &mut SweepCache,
         services: &mut Services,
     ) -> Self::Swept {
+        let (sketch, surface) = self;
         let path = path.into();
 
         let mut shells = Vec::new();
-        for face in self.0.faces(self.1, services) {
+        for face in sketch.faces(surface, services) {
             let shell = face.sweep_with_cache(path, cache, services);
             shells.push(shell);
         }

From 24e38b806eda5ce6cb5224a9659c08f315fe2a03 Mon Sep 17 00:00:00 2001
From: Hanno Braun <hanno@braun-odw.eu>
Date: Thu, 21 Sep 2023 11:14:52 +0200
Subject: [PATCH 13/14] Inline `Sketch::faces` into its only use site

---
 crates/fj-core/src/algorithms/sweep/sketch.rs |  6 ++++--
 crates/fj-core/src/objects/kinds/sketch.rs    | 17 +----------------
 2 files changed, 5 insertions(+), 18 deletions(-)

diff --git a/crates/fj-core/src/algorithms/sweep/sketch.rs b/crates/fj-core/src/algorithms/sweep/sketch.rs
index 6b95c04b1..fe026e8e4 100644
--- a/crates/fj-core/src/algorithms/sweep/sketch.rs
+++ b/crates/fj-core/src/algorithms/sweep/sketch.rs
@@ -1,7 +1,7 @@
 use fj_math::Vector;
 
 use crate::{
-    objects::{Sketch, Solid, Surface},
+    objects::{Face, Sketch, Solid, Surface},
     operations::Insert,
     services::Services,
     storage::Handle,
@@ -22,7 +22,9 @@ impl Sweep for (Handle<Sketch>, Handle<Surface>) {
         let path = path.into();
 
         let mut shells = Vec::new();
-        for face in sketch.faces(surface, services) {
+        for region in sketch.regions() {
+            let face =
+                Face::new(surface.clone(), region.clone()).insert(services);
             let shell = face.sweep_with_cache(path, cache, services);
             shells.push(shell);
         }
diff --git a/crates/fj-core/src/objects/kinds/sketch.rs b/crates/fj-core/src/objects/kinds/sketch.rs
index 8deb53d9a..3709783fd 100644
--- a/crates/fj-core/src/objects/kinds/sketch.rs
+++ b/crates/fj-core/src/objects/kinds/sketch.rs
@@ -1,7 +1,5 @@
 use crate::{
-    objects::{handles::HandleSet, Face, FaceSet, HandleIter, Region, Surface},
-    operations::Insert,
-    services::Services,
+    objects::{handles::HandleSet, HandleIter, Region},
     storage::Handle,
 };
 
@@ -23,17 +21,4 @@ impl Sketch {
     pub fn regions(&self) -> HandleIter<Region> {
         self.regions.iter()
     }
-
-    /// Apply the regions of the sketch to some [`Surface`]
-    pub fn faces(
-        &self,
-        surface: Handle<Surface>,
-        services: &mut Services,
-    ) -> FaceSet {
-        self.regions()
-            .map(|region| {
-                Face::new(surface.clone(), region.clone()).insert(services)
-            })
-            .collect()
-    }
 }

From c48449d500268d985d0eb2b76b6ca86c3b3ea77c Mon Sep 17 00:00:00 2001
From: Hanno Braun <hanno@braun-odw.eu>
Date: Thu, 21 Sep 2023 11:16:29 +0200
Subject: [PATCH 14/14] Remove unused code

---
 crates/fj-core/src/objects/kinds/face.rs | 47 ------------------------
 crates/fj-core/src/objects/mod.rs        |  2 +-
 2 files changed, 1 insertion(+), 48 deletions(-)

diff --git a/crates/fj-core/src/objects/kinds/face.rs b/crates/fj-core/src/objects/kinds/face.rs
index 1a8df0b72..f97b4145d 100644
--- a/crates/fj-core/src/objects/kinds/face.rs
+++ b/crates/fj-core/src/objects/kinds/face.rs
@@ -1,5 +1,3 @@
-use std::collections::{btree_set, BTreeSet};
-
 use fj_math::Winding;
 
 use crate::{
@@ -71,51 +69,6 @@ impl Face {
     }
 }
 
-/// A collection of faces
-#[derive(Clone, Debug, Default, Eq, PartialEq, Hash, Ord, PartialOrd)]
-pub struct FaceSet {
-    inner: BTreeSet<Handle<Face>>,
-}
-
-impl FaceSet {
-    /// Create an empty instance of `Faces`
-    pub fn new() -> Self {
-        Self::default()
-    }
-}
-
-impl Extend<Handle<Face>> for FaceSet {
-    fn extend<T: IntoIterator<Item = Handle<Face>>>(&mut self, iter: T) {
-        self.inner.extend(iter);
-    }
-}
-
-impl FromIterator<Handle<Face>> for FaceSet {
-    fn from_iter<T: IntoIterator<Item = Handle<Face>>>(iter: T) -> Self {
-        let mut faces = Self::new();
-        faces.extend(iter);
-        faces
-    }
-}
-
-impl IntoIterator for FaceSet {
-    type Item = Handle<Face>;
-    type IntoIter = btree_set::IntoIter<Handle<Face>>;
-
-    fn into_iter(self) -> Self::IntoIter {
-        self.inner.into_iter()
-    }
-}
-
-impl<'a> IntoIterator for &'a FaceSet {
-    type Item = &'a Handle<Face>;
-    type IntoIter = btree_set::Iter<'a, Handle<Face>>;
-
-    fn into_iter(self) -> Self::IntoIter {
-        self.inner.iter()
-    }
-}
-
 /// The handedness of a face's coordinate system
 ///
 /// See [`Face::coord_handedness`].
diff --git a/crates/fj-core/src/objects/mod.rs b/crates/fj-core/src/objects/mod.rs
index c69fb8707..0f170f05d 100644
--- a/crates/fj-core/src/objects/mod.rs
+++ b/crates/fj-core/src/objects/mod.rs
@@ -51,7 +51,7 @@ pub use self::{
         curve::Curve,
         cycle::Cycle,
         edge::Edge,
-        face::{Face, FaceSet, Handedness},
+        face::{Face, Handedness},
         region::Region,
         shell::Shell,
         sketch::Sketch,