wjbr

    define fibfast(n)  ; Typical function notation
      if {n < 2}       ; Indentation, infix {...}
        n              ; Single expr = no new list
        fibup(n 2 1 0) ; Simple function calls

    define fibup(maxnum count n-1 n-2)
      if {maxnum = count}
        {n-1 + n-2}
        fibup maxnum {count + 1} {n-1 + n-2} n-1

    define factorial(n)
      if {n <= 1}
        1
        {n * factorial{n - 1}} ; f{...} => f({...})

  (+ 1 2 3)