diff -r f7b87d84864d -r 4f6ca107ccb1 src/direct_product.rs
--- a/src/direct_product.rs	Sun Dec 22 14:54:46 2024 -0500
+++ b/src/direct_product.rs	Sun Dec 22 15:30:34 2024 -0500
@@ -267,7 +267,8 @@
         self.0.norm2_squared() + self.1.norm2_squared()
     }
 
-    fn dist2_squared(&self, Pair(ref u, ref v) : &Self) -> F {
+    fn dist2_squared<I : Instance<Self>>(&self, other : I) -> F {
+        let Pair(u, v) = other.decompose();
         self.0.dist2_squared(u) + self.1.dist2_squared(v)
     }
 }