docs: public Map<String, String> getProperties() {
          final Map<String, String> res = new HashMap<>();
          for (final Map.Entry<String, Object> e: prop.entrySet()) {
              if (e.getValue() instanceof String) {
                  res.put(e.getKey(), (String) e.getValue());
              }
          }
          return Collections.unmodifiableMap(res);
      }