context Block: (es: List) :=
  Block new: es arguments: [] in: context

context Block: (es: List) arguments: (as: List) :=
  Block new: es arguments: as in: context

(b: Block) in-context :=
  b clone do:
    { call := b call-in: b context
      call: vs := b context join: b with: vs
    }

(start: Integer) to: (end: Integer) by: (diff: Integer) do: b :=
  { cc |
    n = start

    while: { (n - diff - end) abs >= diff abs } do: {
      b call: n
      n = n + diff
    }
  } call/cc

(start: Integer) up-to: (end: Integer) do: b :=
  start to: end by: 1 do: b

(start: Integer) down-to: (end: Integer) do: b :=
  start to: end by: -1 do: b

(n: Integer) times: (b: Block) :=
  1 up-to: n do: b in-context