@Override
    public String toString() {
      return description;
    }
  }

  /** Implementation of {@link #digit()}. */
  private static final class Digit extends RangesMatcher {
    // Plug the following UnicodeSet pattern into
    // https://unicode.org/cldr/utility/list-unicodeset.jsp
    // [[:Nd:]&[:nv=0:]&[\u0000-\uFFFF]]
    // and get the zeroes from there.

    // Must be in ascending order.

    @Override
    public String toString() {
      return description;
    }
  }

  /** Implementation of {@link #digit()}. */
  private static final class Digit extends RangesMatcher {
    // Plug the following UnicodeSet pattern into
    // https://unicode.org/cldr/utility/list-unicodeset.jsp
    // [[:Nd:]&[:nv=0:]&[\u0000-\uFFFF]]
    // and get the zeroes from there.

    // Must be in ascending order.