@Override
  public Path dir(String dir, Closure<?> dirSpec) throws IOException {
    Path result = dir(dir);
    callSpec(dirSpec, result);
    return result;
  }
Basically the same as {@link DirectoryFixture#dir(String, Closure)}, but with the current root as base.
spock-core/src/main/java/spock/util/io/FileSystemFixture.java