This is what I have done , I Guess I am missing something ; This function takes a list of symbols and list of numbers and produces all possible pairs of numbers and symbols ; Data definition A list of symbols is either an empty or a los , lon is either empty or lon ; contract : cross los * lon - list of lists ; Purpose : for every item in the los make a list with every item in lon and make a list of all these lists ;examples : (cross '(a b c) '(1 2)) ;(list (list 'a 1) (list 'a 2) (list 'b 1) (list 'b 2) (list 'c 1) (list 'c 2)) ;template : - (define cross (first los ) ( rest los) (first lon ) (rest lon)) (define cross (lambda ( los lon) (cond [(or (empty? los)(empty? lon)) empty] [else ((cr (first los) lon)(cross (rest los) lon))]))) ;contract symbol * lon -> list list of symbols and number ;purpose:-take the symbol and make a list of list of symbol and every element of lon ;examples : (list (list a 1) (list a 2)) (define cr (lambda (sym lon) (cond [(empty? lon) empty] [else (append (list sym (first lon))(list(cr sym (rest lon)))) ]))) ;(cr 'a '(1 2)) (cross '(a) '(1)) ;(cross '(a b c) '()) procedure application: expected procedure, given: (list 'a 1 empty); arguments were: empty Thanks Aditya