// foldr: right fold is a classic higher order function
//       It runs a function over each element in a list.
//
//  lst: pair list, the input
//  fun: processes one element in the list
//  acc: the accumulated result, the output
//
(defn foldr [lst fun acc]
    (cond                  // cond is zygo's if-then-else.
        (empty? lst) acc   // return acculated output if no more input.
            (fun           // else call fun on the head of lst
               (car lst)      
               (foldr (cdr lst) fun acc)) // recursive call on the tail of the input
    )
)