Added additional doctest and fixed indices (#141)

src/optimization/mod.rs

@@ -5,7 +5,7 @@ pub type F<'a, T, X> = dyn for<'b> Fn(&'b X) -> T + 'a;
 pub type DF<'a, X> = dyn for<'b> Fn(&'b mut X, &'b X) + 'a;
 
 #[allow(clippy::upper_case_acronyms)]
-#[derive(Debug, PartialEq)]
+#[derive(Debug, PartialEq, Eq)]
 pub enum FunctionOrder {
     SECOND,
     THIRD,