checkForConcurrentModification();
      if (next == null) {
        throw new NoSuchElementException();
      }
      current = next;
      seenKeys.add(current.key);
      do { // skip ahead to next unseen key
        next = next.next;
      } while ((next != null) && !seenKeys.add(next.key));
      return current.key;
    }

    @Override
    public void remove() {

      case 1:
        return is(sequence.charAt(0));
      case 2:
        return isEither(sequence.charAt(0), sequence.charAt(1));
      default:
        // TODO(lowasser): is it potentially worth just going ahead and building a precomputed
        // matcher?
        return new AnyOf(sequence);
    }
  }

  /**
   * Returns a {@code char} matcher that matches any BMP character not present in the given

      case 1:
        return is(sequence.charAt(0));
      case 2:
        return isEither(sequence.charAt(0), sequence.charAt(1));
      default:
        // TODO(lowasser): is it potentially worth just going ahead and building a precomputed
        // matcher?
        return new AnyOf(sequence);
    }
  }

  /**
   * Returns a {@code char} matcher that matches any BMP character not present in the given

   * ensure type safety. For best results, use the standard method-chaining idiom illustrated in the
   * class documentation above, configuring a builder and building your cache in a single statement.
   * Failure to heed this advice can result in a {@link ClassCastException} being thrown by a cache
   * operation at some <i>undefined</i> point in the future.
   *

   * ensure type safety. For best results, use the standard method-chaining idiom illustrated in the
   * class documentation above, configuring a builder and building your cache in a single statement.
   * Failure to heed this advice can result in a {@link ClassCastException} being thrown by a cache
   * operation at some <i>undefined</i> point in the future.
   *