gcd := vals -> {
  () -> {
    (a, b) := vals
    if (a, b).eq
      (a)
    else if ((a, b).diff.signum, 1).eq
      &vals = (a, (a, b).diff)
    else
      &vals = ((b, a).diff, b)
  }
}.loop

get_num := () -> {
  line := ().read_line.trim
  (
    line.parse_float,
    (
      () -> {
        ("error: '", line, "' not a number!").concat.println
        1.panic
      }
      n -> n,
    )
  ).try
}

("gcd of 899 and 2900 is ", (899, 2900).gcd).concat.println // 29
"Now type two numbers!".println
(a, b) := (().get_num, ().get_num)
("gcd of ", a, " and ", b, " is ", (a, b).gcd).concat.println