diff -r 9327d544ca0b -r 15bfebfbfa3e src/linops.rs
--- a/src/linops.rs	Sun Dec 22 17:24:33 2024 -0500
+++ b/src/linops.rs	Sun Dec 22 17:42:14 2024 -0500
@@ -184,6 +184,93 @@
 }
 
 
+/// The zero operator
+#[derive(Clone,Copy,Debug,Serialize,Eq,PartialEq)]
+pub struct ZeroOp<'a, X, XD, Y, F> {
+    zero : &'a Y, // TODO: don't pass this in `new`; maybe not even store.
+    dual_or_predual_zero : XD,
+    _phantoms : PhantomData<(X, Y, F)>,
+}
+
+// TODO: Need to make Zero in Instance.
+
+impl<'a, F : Num, X : Space, XD, Y : Space + Clone> ZeroOp<'a, X, XD, Y, F> {
+    pub fn new(zero : &'a Y, dual_or_predual_zero : XD) -> Self {
+        ZeroOp{ zero, dual_or_predual_zero, _phantoms : PhantomData }
+    }
+}
+
+impl<'a, F : Num, X : Space, XD, Y : AXPY<F> + Clone> Mapping<X> for ZeroOp<'a, X, XD, Y, F> {
+    type Codomain = Y;
+
+    fn apply<I : Instance<X>>(&self, _x : I) -> Y {
+        self.zero.clone()
+    }
+}
+
+impl<'a, F : Num, X : Space, XD, Y : AXPY<F> + Clone> Linear<X> for ZeroOp<'a, X, XD, Y, F>
+{ }
+
+#[replace_float_literals(F::cast_from(literal))]
+impl<'a, F, X, XD, Y> GEMV<F, X, Y> for ZeroOp<'a, X, XD, Y, F>
+where
+    F : Num,
+    Y : AXPY<F, Y> + Clone,
+    X : Space
+{
+    // Computes  `y = αAx + βy`, where `A` is `Self`.
+    fn gemv<I : Instance<X>>(&self, y : &mut Y, _α : F, _x : I, β : F) {
+        *y *= β;
+    }
+
+    fn apply_mut<I : Instance<X>>(&self, y : &mut Y, _x : I){
+        y.set_zero();
+    }
+}
+
+impl<'a, F, X, XD, Y, E1, E2> BoundedLinear<X, E1, E2, F> for ZeroOp<'a, X, XD, Y, F>
+where
+    X : Space + Norm<F, E1>,
+    Y : AXPY<F> + Clone + Norm<F, E2>,
+    F : Num,
+    E1 : NormExponent,
+    E2 : NormExponent,
+{
+    fn opnorm_bound(&self, _xexp : E1, _codexp : E2) -> F { F::ZERO }
+}
+
+impl<'a, F : Num, X, XD, Y, Yprime : Space> Adjointable<X, Yprime> for ZeroOp<'a, X, XD, Y, F>
+where
+     X : Space,
+     Y :  AXPY<F> + Clone + 'static,
+     XD : AXPY<F> + Clone + 'static,
+{
+    type AdjointCodomain = XD;
+    type Adjoint<'b> = ZeroOp<'b, Yprime, (), XD, F> where Self : 'b;
+        // () means not (pre)adjointable.
+
+    fn adjoint(&self) -> Self::Adjoint<'_> {
+        ZeroOp::new(&self.dual_or_predual_zero, ())
+    }
+}
+
+impl<'a, F, X, XD, Y, Ypre> Preadjointable<X, Ypre> for ZeroOp<'a, X, XD, Y, F>
+where
+    F : Num,
+    X : Space,
+    Y : AXPY<F> + Clone,
+    Ypre : Space,
+    XD : AXPY<F> + Clone + 'static,
+{
+    type PreadjointCodomain = XD;
+    type Preadjoint<'b> = ZeroOp<'b, Ypre, (), XD, F> where Self : 'b;
+        // () means not (pre)adjointable.
+
+    fn preadjoint(&self) -> Self::Preadjoint<'_> {
+        ZeroOp::new(&self.dual_or_predual_zero, ())
+    }
+}
+
 impl<S, T, E, X> Linear<X> for Composition<S, T, E>
 where
     X : Space,