return BASE16;
  }

  static final class Alphabet {
    private final String name;
    // this is meant to be immutable -- don't modify it!
    private final char[] chars;
    final int mask;
    final int bitsPerChar;
    final int charsPerChunk;
    final int bytesPerChunk;
    private final byte[] decodabet;
    private final boolean[] validPadding;
    private final boolean ignoreCase;