diff -r d14c877e14b7 -r b3c35d16affe src/bisection_tree/either.rs
--- a/src/bisection_tree/either.rs	Tue Feb 20 12:33:16 2024 -0500
+++ b/src/bisection_tree/either.rs	Mon Feb 03 19:22:16 2025 -0500
@@ -3,8 +3,14 @@
 use std::sync::Arc;
 
 use crate::types::*;
-use crate::mapping::Apply;
-use crate::iter::{Mappable,MapF,MapZ};
+use crate::mapping::{
+    Instance,
+    Mapping,
+    DifferentiableImpl,
+    DifferentiableMapping,
+    Space,
+};
+use crate::iter::{Mappable, MapF, MapZ};
 use crate::sets::Cube;
 use crate::loc::Loc;
 
@@ -177,12 +183,17 @@
     }
 }
 
-impl<F, S1, S2, X> Apply<X> for EitherSupport<S1, S2>
-where S1 : Apply<X, Output=F>,
-      S2 : Apply<X, Output=F> {
-    type Output = F;
+impl<F, S1, S2, X> Mapping<X> for EitherSupport<S1, S2>
+where
+    F : Space,
+    X : Space,
+    S1 : Mapping<X, Codomain=F>,
+    S2 : Mapping<X, Codomain=F>,
+{
+    type Codomain = F;
+
     #[inline]
-    fn apply(&self, x : X) -> F {
+    fn apply<I : Instance<X>>(&self, x : I) -> F {
         match self {
             EitherSupport::Left(ref a) => a.apply(x),
             EitherSupport::Right(ref b) => b.apply(x),
@@ -190,6 +201,24 @@
     }
 }
 
+impl<X, S1, S2, O> DifferentiableImpl<X> for EitherSupport<S1, S2>
+where
+    O : Space,
+    X : Space,
+    S1 : DifferentiableMapping<X, DerivativeDomain=O>,
+    S2 : DifferentiableMapping<X, DerivativeDomain=O>,
+{
+    type Derivative = O;
+
+    #[inline]
+    fn differential_impl<I : Instance<X>>(&self, x : I) -> O {
+        match self {
+            EitherSupport::Left(ref a) => a.differential(x),
+            EitherSupport::Right(ref b) => b.differential(x),
+        }
+    }
+}
+
 macro_rules! make_either_scalarop_rhs {
     ($trait:ident, $fn:ident, $trait_assign:ident, $fn_assign:ident) => {
         impl<F : Float, G1, G2>