code: @Cached
      @DontLabel
      public Rule anyOf(final Characters characters)
      {
          Objects.requireNonNull(characters);
          if (!characters.isSubtractive() && characters.getChars().length == 1)
              return ch(characters.getChars()[0]);
          if (characters.equals(Characters.NONE))
              return NOTHING;
          return new AnyOfMatcher(characters);
      }
docs: Match any given character among a set of characters
  
  <p>Both {@link #anyOf(char[])} and {@link #anyOf(String)} ultimately
  delegate to this method, which caches its resuls.</p>
  
  @param characters the characters
  @return a new rule