Code formatted:

  REBOL [title: "Calculator"]
  view layout [
      origin 0  space 0x0  across
      style btn btn 50x50 [append f/text face/text  show f]
      f: field 200x40 font-size 20 return
      btn "1"  btn "2"  btn "3"  btn " + "  return
      btn "4"  btn "5"  btn "6"  btn " - "  return
      btn "7"  btn "8"  btn "9"  btn " * "  return
      btn "0"  btn "."  btn " / "   btn "=" [
          attempt [f/text: form do f/text  show f]
      ]
  ]