code: public static Set<String> getKeysIn(final Map map, final Object... path) {
          return getMap(map, path)
              .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 path nodes to walk in map
  @return value