procedure fib(n)
   local  source, i
   if type(n) == "integer" & n >= 0 then
      return n @ makeProc {{
         i := @(source := &source)
         if i = (0|1) then i@source
         ((i-1)@makeProc(^&current) + (i-2)@makeProc(^&current)) @ source
         }}
end