add difference::Diff alias
@@ -10,6 +10,8 @@ use std::ops::{Add, Sub, Neg, Mul};
use ::Data;
+pub use self::Abelian as Diff;
+
/// A type that can be treated as a difference.
///
/// The mathematical requirements are, I believe, an Abelian group, in that we require addition, inverses,
