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