diff -r 4e09b7829b51 -r f40dfaf2166d src/bisection_tree/bt.rs
--- a/src/bisection_tree/bt.rs	Tue Nov 01 09:24:45 2022 +0200
+++ b/src/bisection_tree/bt.rs	Fri Nov 18 10:29:50 2022 +0200
@@ -109,7 +109,7 @@
     fn lower_or(&self) -> Self::Lower;
 
     /// Returns the numeric value of the depth
-    fn value(&self) -> usize;
+    fn value(&self) -> u32;
 }
 
 /// Dynamic (runtime) [`Depth`] for a [`BT`].
@@ -136,8 +136,8 @@
     }
 
     #[inline]
-    fn value(&self) -> usize {
-        self.0 as usize
+    fn value(&self) -> u32 {
+        self.0 as u32
     }
 }
 
@@ -145,7 +145,7 @@
     type Lower = Self;
     fn lower(&self) -> Option<Self::Lower> { None }
     fn lower_or(&self) -> Self::Lower { Const }
-    fn value(&self) -> usize { 0 }
+    fn value(&self) -> u32 { 0 }
 }
 
 macro_rules! impl_constdepth {
@@ -154,7 +154,7 @@
             type Lower = Const<{$n-1}>;
             fn lower(&self) -> Option<Self::Lower> { Some(Const) }
             fn lower_or(&self) -> Self::Lower { Const }
-            fn value(&self) -> usize { $n }
+            fn value(&self) -> u32 { $n }
         }
     )* };
 }
@@ -474,9 +474,9 @@
             NodeOption::Branches(b) => {
                 // FIXME: recursion that may cause stack overflow if the tree becomes
                 // very deep, e.g. due to [`BTSearch::search_and_refine`].
-                Arc::make_mut(b).insert(domain, d, new_leaf_depth.lower_or(), support,
-                                        task_budget);
-                b.summarise_into(&mut self.aggregator)
+                let bm = Arc::make_mut(b);
+                bm.insert(domain, d, new_leaf_depth.lower_or(), support, task_budget);
+                bm.summarise_into(&mut self.aggregator);
             },
         }
     }
@@ -517,11 +517,11 @@
             NodeOption::Branches(b) => {
                 // FIXME: recursion that may cause stack overflow if the tree becomes
                 // very deep, e.g. due to [`BTSearch::search_and_refine`].
-                let bnew = Arc::new(Arc::unwrap_or_clone(b).convert_aggregator(generator, domain));
+                let bnew = Arc::unwrap_or_clone(b).convert_aggregator(generator, domain);
                 let mut anew = ANew::new();
                 bnew.summarise_into(&mut anew);
                 Node {
-                    data : NodeOption::Branches(bnew),
+                    data : NodeOption::Branches(Arc::new(bnew)),
                     aggregator : anew,
                 }
             }
@@ -551,8 +551,9 @@
             NodeOption::Branches(ref mut b) => {
                 // FIXME: recursion that may cause stack overflow if the tree becomes
                 // very deep, e.g. due to [`BTSearch::search_and_refine`].
-                Arc::make_mut(b).refresh_aggregator(generator, domain, task_budget);
-                b.summarise_into(&mut self.aggregator);
+                let bm = Arc::make_mut(b);
+                bm.refresh_aggregator(generator, domain, task_budget);
+                bm.summarise_into(&mut self.aggregator);
             }
         }
     }