code: private Stream<Object> flatten(final Object o) {
          if (o instanceof Map<?, ?>) {
              return ((Map<?, ?>) o).values().stream().flatMap(this::flatten);
          }
          return Stream.of(o);
      }
docs: Recursively go along yaml nodes beneath the given one to flatten string values
  @param o YAML node point of start
  @return flattened values beneath given YAML node