// Bind : is faster than assignment :=
// loop is faster than to_pre (which uses a closure)
() Integer
  [
  if this < 2
    [this]
  else
    [
    !prev: 1
    !next: 1
    !sum
    !count: this - 2
    loop
      [
      if count = 0 [exit]
      count--
      sum  : prev + next
      prev : next
      next : sum
      ]

    next
    ]
  ]