diff -r 21e51de02ab6 -r b071a1b484f8 src/linops.rs
--- a/src/linops.rs	Fri Sep 05 00:16:08 2025 -0500
+++ b/src/linops.rs	Fri Sep 05 00:48:59 2025 -0500
@@ -44,9 +44,13 @@
 
     /// Principal form of the space; always equal to [`Space::Principal`], but with
     /// more traits guaranteed.
+    ///
+    /// `PrincipalV` is only assumed to be `AXPY` for itself, as [`AXPY`]
+    /// uses [`Instance`] to apply all other variants and avoid problems
+    /// of choosing multiple implementations of the trait.
     type PrincipalV: ClosedSpace
         + AXPY<
-            Self,
+            Self::PrincipalV,
             Field = Self::Field,
             PrincipalV = Self::PrincipalV,
             OwnedVariant = Self::PrincipalV,