Add comment to acyclicgraph API

1 files changed, 4 insertions, 1 deletions

diff --git a/lib/acyclicGraph.mli b/lib/acyclicGraph.mli
index 13a12b51cd..b53a4c018f 100644
--- a/lib/acyclicGraph.mli
+++ b/lib/acyclicGraph.mli
@@ -39,10 +39,13 @@ module Make (Point:Point) : sig
 
   exception AlreadyDeclared
   val add : ?rank:int -> Point.t -> t -> t
-  (** Use a large [rank] to keep the node canonical *)
+  (** All points must be pre-declared through this function before
+     they can be mentioned in the others. NB: use a large [rank] to
+     keep the node canonical *)
 
   exception Undeclared of Point.t
   val check_declared : t -> Point.Set.t -> unit
+  (** @raise Undeclared if one of the points is not present in the graph. *)
 
   type 'a check_function = t -> 'a -> 'a -> bool