docs: public ConfigResolveOptions appendResolver(ConfigResolver value) {
          if (value == null) {
              throw new ConfigException.BugOrBroken("null resolver passed to appendResolver");
          } else if (value == this.resolver) {
              return this;
          } else {
              return new ConfigResolveOptions(useSystemEnvironment, allowUnresolved,
                      this.resolver.withFallback(value));
          }
      }