Highlight some hard to handle edge-cases in fn:chain since the arity of functions is set but return sequences can vary in size

chain.xq

let $xml :=
  <root>
    <child>1</child>
    <child>2</child>
    <child>3</child>
    <!--<child>4</child>-->
  </root>

let $children := fn ($n) { $n/child }

return (
  chain($xml/root, (
    $children,
    fn ($a, $b, $c) { concat($a, $b, $c) }
  )),
  chain($xml/root, (
    $children,
    tail#1,
    fn ($a, $b, $c) { concat($a, $b, $c) }
  )),
  chain($xml/root, (
    $children,
    concat#2
  )),
  chain($xml/root, (
    $children,
    apply(concat#3, ?)
  ))
)