code: public Optional<List<String>> getList(String key) {
      return this.get(key).map(v -> ImmutableList.copyOf(Splitter.on(',').split(v)));
    }
docs: Gets the flag value for the given key as a comma-separated {@link List} of Strings, wrapped in
  an {@link Optional}, which is empty if the flag is unset.
  
  <p>(note: empty strings included, e.g. {@code "-XepOpt:List=,1,,2," => ["","1","","2",""]})