Fixed with more trait bounds on Number

src/lib.rs

@@ -6,7 +6,13 @@ type Float = f32;
 
 pub trait Number
 where
-    Self: std::marker::Sized + Copy + fmt::Display + Mul + Sub,
+    Self: std::marker::Sized
+        + Copy
+        + fmt::Display
+        + Mul
+        + Mul<Output = Self>
+        + Sub
+        + Sub<Output = Self>,
 {
     // Real version has stuff here
 }
@@ -83,11 +89,7 @@ where
     T: Number,
 {
     pub fn area(&self) -> T {
-        // TODO(glenng) make this compile.
-        /*
         let d = self.p_max - self.p_min;
         d.x * d.y
-         */
-        unimplemented!();
     }
 }