#import nat

fib =

~&izZB?(                    # test the sign bit of the argument
   <'fib of negative'>!%,   # throw an exception if it's negative
   {0,1}^?<a(               # test the argument to a recursively defined function
      ~&a,                  # if the argument was a member of {0,1}, return it
      sum^|W(               # otherwise return the sum of two recursive calls
         ~&,                # to the function thus defined
         predecessor^~(     # with the respective predecessors of
            ~&,             # the given argument
            predecessor)))) # and the predecessor thereof