;; x? over a dyn: true when it is not nil. A dyn x stays a dyn in the block,
;; and e? as g binds a dyn that is present (decision 133).

fn describe(a, b)
  if a?
    a + 100
  elif b? and b > 5
    b + 200
  else
    0

fn run(a, n)
  println(a?, n?)
  println(describe(a, n), describe(n, 9), describe(n, 2))
  let m = {:name "ann" :pet nil}
  if m.pet? as pet
    println(pet)
  else
    println("no pet")
  if m.name? as who
    println(who)
  let xs = [1, 2, nil, 4]
  let i = 0
  let total = 0
  while get(xs, i)? as x
    total += x
    i += 1
  println(total, i)

fn main()
  run(3, nil)