--- a/rust/hg-core/src/lib.rs	Tue Dec 11 17:31:54 2018 +0100
+++ b/rust/hg-core/src/lib.rs	Fri Nov 30 00:44:04 2018 +0100
@@ -15,7 +15,7 @@
 
 /// The simplest expression of what we need of Mercurial DAGs.
 pub trait Graph {
-    fn parents(&self, Revision) -> Result<(Revision, Revision), GraphError>;
+    fn parents(&self, Revision) -> Result<[Revision; 2], GraphError>;
 }
 
 #[derive(Clone, Debug, PartialEq)]