public TransitionLabel<S, L> getSignatureLetter(int index) {
        if (index < signature.size()) {
            return signature.get(index);
        }
        return null;
    }
public TransitionLabel < S , L > getSignatureLetter ( int index ) { if ( index < signature . size ( ) ) { return signature . get ( index ) ; } return null ; }
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>.