1 files changed, 7 insertions, 3 deletions
diff --git a/sheet.mli b/sheet.mli
index 59a1fa7..d3c8151 100755
--- a/sheet.mli
+++ b/sheet.mli
@@ -17,13 +17,17 @@ module Raw: sig
   val add: cell -> Expression.t -> t -> Cell.Set.t * t
   
   val remove: cell -> t -> Cell.Set.t * t
-  
-  val get_value: cell -> t -> ScTypes.result
+
+  (** Get the value content. 
+      @return None if the cell is not defined
+   *)
+  val get_value: cell -> t -> ScTypes.result option
   
   val get_expr: cell -> t -> Expression.t
 
   val get_sink: cell -> t -> Cell.Set.t
   
+  (** Fold over all the defined values *)
   val fold: ('a -> cell -> (Expression.t * ScTypes.result ) -> 'a) -> 'a -> t -> 'a
 
 end
@@ -38,7 +42,7 @@ type t = {
 }
 
 type search = [
-  | `Pattern of ScTypes.result
+  | `Pattern of ScTypes.result option
   | `Next
   | `Previous
 ]