Add blanket implementation for (D1, D2) when they impl Dimension

zed/src/sum_tree.rs

@@ -36,6 +36,18 @@ impl<'a, T: Summary> Dimension<'a, T> for () {
     fn add_summary(&mut self, _: &'a T, _: &T::Context) {}
 }
 
+impl<'a, S, D1, D2> Dimension<'a, S> for (D1, D2)
+where
+    S: Summary,
+    D1: Dimension<'a, S>,
+    D2: Dimension<'a, S>,
+{
+    fn add_summary(&mut self, summary: &'a S, cx: &S::Context) {
+        self.0.add_summary(summary, cx);
+        self.1.add_summary(summary, cx);
+    }
+}
+
 pub trait SeekDimension<'a, T: Summary>: Dimension<'a, T> {
     fn cmp(&self, other: &Self, cx: &T::Context) -> Ordering;
 }