code: public static <I> GrowingAlphabet<I> toGrowingAlphabetOrThrowException(final Alphabet<I> alphabet)
              throws GrowingAlphabetNotSupportedException {
          if (alphabet instanceof GrowingAlphabet) {
              return (GrowingAlphabet<I>) alphabet;
          } else {
              throw new GrowingAlphabetNotSupportedException(alphabet);
          }
      }
docs: Casts the given alphabet to a {@link GrowingAlphabet} if possible (i.e. the given alphabet is actually an
  instance of a {@link GrowingAlphabet}). Throws a {@link GrowingAlphabetNotSupportedException} otherwise.
  
  @param alphabet
  the alphabet to cast
  @param <I>
  input symbol type
  
  @return the same alphabet instance, casted to a {@link GrowingAlphabet}.
  
  @throws GrowingAlphabetNotSupportedException
  if the given alphabet is not an instance of {@link GrowingAlphabet}.