private static void checkNormalizationNotSet(
        PathNormalization n, @Nullable PathNormalization set) {
      if (set != null) {
        throw new IllegalArgumentException(
            "can't set normalization " + n + ": normalization " + set + " already set");
      }
    }
Returns the normalizations that will be applied to the canonical form of filenames in the
file system. The canonical form is used to determine the equality of two filenames when
performing a file lookup.
jimfs/src/main/java/com/google/common/jimfs/Configuration.java