static L flattenCollectionsAndArrays(Iterable a) {
  new L l;
  for (O x : a)
    if (x cast Cl)
      l.addAll(flattenCollectionsAndArrays(x));
    else if (x cast O[])
      l.addAll(flattenCollectionsAndArrays(asList(x)));
    else
      l.add(x);
  ret l;
}

static L flattenCollectionsAndArrays(O... whatever) {
  ret flattenCollectionsAndArrays(ll(whatever));
}

Began life as a copy of #1032754