--- a/src/instance.rs	Sun Jan 19 16:49:09 2025 +0100
+++ b/src/instance.rs	Sun Jan 19 22:33:21 2025 +0100
@@ -89,6 +89,8 @@
 /// Helper trait for functions to work with either owned values or references to either the
 /// “principal type” `X` or types some present a subset of `X`. In the latter sense, this
 /// generalises [`std::borrow::ToOwned`], [`std::borrow::Borrow`], and [`std::borrow::Cow`].
+///
+/// This is used, for example, by [`crate::mapping::Mapping::apply`].
 pub trait Instance<X : Space, D = <X as Space>::Decomp> : Sized where D : Decomposition<X> {
     /// Decomposes self according to `decomposer`.
     fn decompose<'b>(self) -> D::Decomposition<'b>