code: public TransitionLabel<S, L> getSignatureLetter(int index) {
          if (index < signature.size()) {
              return signature.get(index);
          }
          return null;
      }
docs: Retrieves the letter from the signature with the given index. If there is no such index (because the signature is
  shorter), <code>null</code> is returned.
  
  @param index
  the signature index.
  
  @return the respective letter of the signature, or <code>null</code>.