[plt-scheme] Typed Scheme: Is there any easy way around this?

(pdefine: (a b) (mappend (fun : (a -> (Listof b))) (lst : (Listof a))) : (Listof b) 
     ;;Append the results of calling fn on each element of list.
    (apply append (map fun lst)))

--> typecheck: polymorphic functions not supported with apply 

-- 
"You do not really understand something unless you can
 explain it to your grandmother." - Albert Einstein.