code: public boolean addToSignature(TransitionLabel<S, L> letter) {
          boolean first = signature.isEmpty();
          signature.add(letter);
          return first;
      }
docs: Adds a transition label (letter) to this state's signature.
  
  @param letter
  the letter to add.
  
  @return <code>true</code> iff this was the first letter to be added to the signature, <code>false</code>
  otherwise.