// TODO: why doesn't it extend ReversibleFunctionImpl?

static class FJoin extends FunctionImpl {
  String glue;
  
  *(String *glue) {}
  
  public O process(Object _in) {
    return join(glue, (List) _in);
  }
  
  public O unprocess(Object _in) {
    String in = cast _in;
    return new ArrayList<String>(Arrays.asList(in.split(glue)));
  }
}

Began life as a copy of #1000510