It works!

src/gaussian.rs

@@ -1,11 +1,19 @@
+use std::fmt;
 use std::ops;
 
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy)]
 pub struct Gaussian {
     pi: f64,
     tau: f64,
 }
 
+impl fmt::Debug for Gaussian {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "N(mu={:.03}, sigma={:.03})", self.mu(), self.sigma())
+    }
+}
+
+
 impl Gaussian {
     pub fn from_pi_tau(pi: f64, tau: f64) -> Gaussian {
         Gaussian { pi, tau }