diff --git a/src/Gyehoek/Scheme/Syntax.hs b/src/Gyehoek/Scheme/Syntax.hs
index 83d1234..6b05656 100644
--- a/src/Gyehoek/Scheme/Syntax.hs
+++ b/src/Gyehoek/Scheme/Syntax.hs
@@ -118,6 +118,12 @@ data Def
   deriving stock (Show, Generic, Data)
   deriving anyclass (NFData)
 
+data Formals a
+  = FormalsFixed (List a)
+  | FormalsVar (List a) a
+  deriving stock (Show, Generic, Data, Functor, Foldable, Traversable)
+  deriving anyclass (NFData)
+
 data Exp
   = ExpLet (List (Name, Exp)) Exp
   | ExpLetSyntax (List (Name, Trans)) Exp