1 files changed, 2 insertions, 0 deletions
diff --git a/lib/ast.ml b/lib/ast.ml
index 9674664..61ca787 100644
--- a/lib/ast.ml
+++ b/lib/ast.ml
@@ -15,6 +15,8 @@ type node =
   | Block of string * (string * node) list  (** [[Rel l: n ...]] *)
   | And of node * node
   | Or of node * node
+  | Not of node * node
+      (** [u :not d]: within the universe [u], deny [d]. *)
 
 (** A program is a list of statements. A bare block asserts a fact;
     a directive is a top-level message send such as