static ItIt mapLike multiCall(O f, O... args) {
  int n = l(args);
  Iterable<O>[] lists = new Iterable[n];
  for i to n: {
    O arg = args[i];
    if (arg cast Multi)
      lists[i] = arg.arguments;
    else
      lists[i] = ll(arg);
  }
  ret mapI(outerProduct_arrays(lists), args2 -> callF(f, args2));
}