diff -r cc68841e758f -r 00029c20c0ee src/bisection_tree/either.rs
--- a/src/bisection_tree/either.rs	Fri Apr 28 08:27:17 2023 +0300
+++ b/src/bisection_tree/either.rs	Fri Apr 28 08:32:15 2023 +0300
@@ -3,8 +3,8 @@
 use std::sync::Arc;
 
 use crate::types::*;
-use crate::mapping::Apply;
-use crate::iter::{Mappable,MapF,MapZ};
+use crate::mapping::{Apply, Differentiate};
+use crate::iter::{Mappable, MapF, MapZ};
 use crate::sets::Cube;
 use crate::loc::Loc;
 
@@ -190,6 +190,19 @@
     }
 }
 
+impl<F, S1, S2, X> Differentiate<X> for EitherSupport<S1, S2>
+where S1 : Differentiate<X, Output=F>,
+      S2 : Differentiate<X, Output=F> {
+    type Output = F;
+    #[inline]
+    fn differential(&self, x : X) -> F {
+        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>