code: public Sql listSeparator(/*@Untainted*/ String sql) {
      if (listFirstItem.peek()) {
        listFirstItem.pop();
        listFirstItem.push(false);
        return this;
      } else {
        return append(sql);
      }
    }
docs: Appends the passed bit of sql only if a previous item has already been appended,
  and notes that the list is not empty.