-- I have replaced Set by Type in order to be more in accordance with what was learned in the 
-- lecture.

data Char = a | b | c | d | e | f | g | h | i | j | k | l | m | n | o | p | q | r | s | t | u | v | w | x | y | z | blank



data String = nil | cons (c:: Char) (s :: String) 

concat (x :: String)
       (y :: String)
       :: String
       = case x of {
           (nil) -> y;
           (cons c x') -> cons c (concat x'  y);}



double  (x:: String)
        :: String
        = {! !}

Test :: Type 
  = {!double (cons h (cons e (cons l (cons l (cons o (cons blank nil))))))  !}