diff --git a/src/ast.ml b/src/ast.ml
index 2179b24..78f1133 100644
--- a/src/ast.ml
+++ b/src/ast.ml
@@ -47,6 +47,7 @@ type t_expression =
   | ETriOp of full_ty * triop * t_expression * t_expression * t_expression
   | EComp  of full_ty * compop * t_expression * t_expression
   | EWhen  of full_ty * t_expression * t_expression
+  | EFby   of full_ty * t_expression * t_expression
   | EConst of full_ty * const
   | ETuple of full_ty * (t_expression list)
   | EApp   of full_ty * t_node * t_expression
diff --git a/src/parser.mly b/src/parser.mly
index 5bc54e9..20632a7 100644
--- a/src/parser.mly
+++ b/src/parser.mly
@@ -421,3 +421,4 @@ ident_comma_list:
   | IDENT COMMA ident_comma_list { $1 :: $3 }
 ;
 
+
diff --git a/src/pp.ml b/src/pp.ml
index fa537f1..035c84e 100644
--- a/src/pp.ml
+++ b/src/pp.ml
@@ -33,6 +33,13 @@ let pp_expression =
       | _ -> raise (MyTypeError "This exception should not have been raised.")
     in
     match expression with
+    | EFby (_, e1, e2) ->
+        begin
+          Format.fprintf fmt "\t\t\t%sFBY\n%a\t\t\tFBY\n%a"
+            prefix
+            (pp_expression_aux (upd_prefix prefix)) e1
+            (pp_expression_aux (upd_prefix prefix)) e2
+        end
     | EWhen (_, e1, e2) ->
         begin
           Format.fprintf fmt "\t\t\t%sWHEN\n%a\t\t\tWHEN\n%a"