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/lexer.mll b/src/lexer.mll
index 5acc6ea..2a4423a 100644
--- a/src/lexer.mll
+++ b/src/lexer.mll
@@ -29,6 +29,7 @@
       ("else", ELSE);
       ("merge", TO_merge);
       ("when", WHEN);
+      ("fby", FBY);
       ("pre", MO_pre);
       ("true", CONST_BOOL(true));
       ("false", CONST_BOOL(false));
diff --git a/src/parser.mly b/src/parser.mly
index fd62e82..33a2420 100644
--- a/src/parser.mly
+++ b/src/parser.mly
@@ -35,6 +35,7 @@
     | ETriOp (full_ty , _ , _ , _ , _) -> full_ty
     | EComp  (full_ty , _ , _ , _) -> full_ty
     | EWhen  (full_ty , _ , _) -> full_ty
+    | EFby   (full_ty , _ , _) -> full_ty
     | EConst (full_ty , _) -> full_ty
     | ETuple (full_ty , _) -> full_ty
     | EApp   (full_ty , _ , _) -> full_ty
@@ -95,6 +96,7 @@
 %token TO_merge
 
 %token WHEN
+%token FBY
 
 %token IF
 %token THEN
@@ -234,6 +236,7 @@ expr:
   | TO_merge expr expr expr            { ETriOp (type_exp $4, TOp_merge, $2, $3, $4) }
   /* When is neither a binop (a * 'a -> 'a) or a comp ('a * 'a -> bool) */
   | expr WHEN expr                     { EWhen (type_exp $1, $1, $3) }
+  | expr FBY expr                      { EFby (type_exp $1, $1, $3) }
   /* Constants */
   | CONST_INT                          { EConst (FTBase TInt, CInt $1) }
   | CONST_BOOL                         { EConst (FTBase TBool, CBool $1) }
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"