Adds a -= to the CuDuration. (#160)

core/cu29_clock/src/lib.rs

@@ -13,7 +13,7 @@ use quanta::{Clock, Mock};
 use serde::{Deserialize, Serialize};
 use std::convert::Into;
 use std::fmt::{Display, Formatter};
-use std::ops::{AddAssign, Div, Mul};
+use std::ops::{AddAssign, Div, Mul, SubAssign};
 use std::sync::Arc;
 use std::time::Duration;
 
@@ -92,6 +92,12 @@ impl AddAssign for CuDuration {
     }
 }
 
+impl SubAssign for CuDuration {
+    fn sub_assign(&mut self, rhs: Self) {
+        self.0 -= rhs.0
+    }
+}
+
 // a way to divide a duration by a scalar.
 // useful to compute averages for example.
 impl<T> Div<T> for CuDuration