diff -r 8783710b1d58 -r dbc28c91f7ff rust/hg-core/src/lib.rs
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hg-core/src/lib.rs	Thu Sep 27 17:03:16 2018 +0200
@@ -0,0 +1,24 @@
+// Copyright 2018 Georges Racinet <gracinet@anybox.fr>
+//
+// This software may be used and distributed according to the terms of the
+// GNU General Public License version 2 or any later version.
+mod ancestors;
+pub use ancestors::AncestorsIterator;
+
+/// Mercurial revision numbers
+///
+/// As noted in revlog.c, revision numbers are actually encoded in
+/// 4 bytes, and are liberally converted to ints, whence the i32
+pub type Revision = i32;
+
+pub const NULL_REVISION: Revision = -1;
+
+/// The simplest expression of what we need of Mercurial DAGs.
+pub trait Graph {
+    fn parents(&self, Revision) -> Result<(Revision, Revision), GraphError>;
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum GraphError {
+    ParentOutOfRange(Revision),
+}