--- a/rust/hg-core/src/lib.rs	Tue Feb 05 10:28:32 2019 +0100
+++ b/rust/hg-core/src/lib.rs	Mon Feb 04 19:46:57 2019 +0100
@@ -13,6 +13,11 @@
 /// 4 bytes, and are liberally converted to ints, whence the i32
 pub type Revision = i32;
 
+
+/// Marker expressing the absence of a parent
+///
+/// Independently of the actual representation, `NULL_REVISION` is guaranteed
+/// to be smaller that all existing revisions.
 pub const NULL_REVISION: Revision = -1;
 
 /// Same as `mercurial.node.wdirrev`