diff -r cc68841e758f -r 00029c20c0ee src/bisection_tree/btfn.rs
--- a/src/bisection_tree/btfn.rs	Fri Apr 28 08:27:17 2023 +0300
+++ b/src/bisection_tree/btfn.rs	Fri Apr 28 08:32:15 2023 +0300
@@ -4,7 +4,7 @@
 use std::marker::PhantomData;
 use std::sync::Arc;
 use crate::types::Float;
-use crate::mapping::{Apply, Mapping};
+use crate::mapping::{Apply, Mapping, Differentiate};
 //use crate::linops::{Apply, Linear};
 use crate::sets::Set;
 use crate::sets::Cube;
@@ -386,10 +386,8 @@
 
 make_btfn_unaryop!(Neg, neg);
 
-
-
 //
-// Mapping
+// Apply, Mapping, Differentiate
 //
 
 impl<'a, F : Float, G, BT, V, const N : usize> Apply<&'a Loc<F, N>>
@@ -422,6 +420,42 @@
     }
 }
 
+impl<'a, F : Float, G, BT, V, const N : usize> Differentiate<&'a Loc<F, N>>
+for BTFN<F, G, BT, N>
+where BT : BTImpl<F, N>,
+      G : SupportGenerator<F, N, Id=BT::Data>,
+      G::SupportType : LocalAnalysis<F, BT::Agg, N> + Differentiate<&'a Loc<F, N>, Output = V>,
+      V : Sum {
+
+    type Output = V;
+
+    fn differential(&self, x : &'a Loc<F, N>) -> Self::Output {
+        self.bt.iter_at(x)
+            .map(|&d| self.generator.support_for(d).differential(x))
+            .sum()
+    }
+}
+
+impl<F : Float, G, BT, V, const N : usize> Differentiate<Loc<F, N>>
+for BTFN<F, G, BT, N>
+where BT : BTImpl<F, N>,
+      G : SupportGenerator<F, N, Id=BT::Data>,
+      G::SupportType : LocalAnalysis<F, BT::Agg, N> + Differentiate<Loc<F, N>, Output = V>,
+      V : Sum {
+
+    type Output = V;
+
+    fn differential(&self, x : Loc<F, N>) -> Self::Output {
+        self.bt.iter_at(&x)
+            .map(|&d| self.generator.support_for(d).differential(x))
+            .sum()
+    }
+}
+
+//
+// GlobalAnalysis
+//
+
 impl<F : Float, G, BT, const N : usize> GlobalAnalysis<F, BT::Agg>
 for BTFN<F, G, BT, N>
 where BT : BTImpl<F, N>,