code: public static Set<String> dotGetKeysIn(final Map map, final String pathString) {
          return dotGetMap(map, pathString)
                  .map(m -> m
                          .keySet()
                          .stream()
                          .map(Object::toString)
                          .collect(Collectors.toSet())
                  ).orElse(new HashSet<>());
      }
docs: Get set of string keys by path.
  @param map subject
  @param pathString nodes to walk in map
  @return value