bring back the example i removed, also add symmetry and simplify impl

src/libcore/cmp.rs

@@ -132,6 +132,48 @@ use self::Ordering::*;
 /// By changing `impl PartialEq for Book` to `impl PartialEq<BookFormat> for Book`,
 /// we allow `BookFormat`s to be compared with `Book`s.
 ///
+/// You can also combine these implementations to let the `==` operator work with
+/// two different types:
+///
+/// ```
+/// #[derive(PartialEq)]
+/// enum BookFormat {
+///     Paperback,
+///     Hardback,
+///     Ebook,
+/// }
+/// 
+/// struct Book {
+///     isbn: i32,
+///     format: BookFormat,
+/// }
+/// 
+/// impl PartialEq<BookFormat> for Book {
+///     fn eq(&self, other: &BookFormat) -> bool {
+///         self.format == *other
+///     }
+/// }
+/// 
+/// impl PartialEq<Book> for BookFormat {
+///     fn eq(&self, other: &Book) -> bool {
+///         *self == other.format
+///     }
+/// }
+/// 
+/// impl PartialEq for Book {
+///     fn eq(&self, other: &Book) -> bool {
+///         self.isbn == other.isbn
+///     }
+/// }
+/// 
+/// let b1 = Book { isbn: 3, format: BookFormat::Paperback };
+/// let b2 = Book { isbn: 3, format: BookFormat::Ebook };
+/// 
+/// assert!(b1 == BookFormat::Paperback);
+/// assert!(b1 != BookFormat::Ebook);
+/// assert!(b1 == b2);
+/// ```
+///
 /// # Examples
 ///
 /// ```