docs: def start_state_id(self):
          """ The start state is the state to which the first transition goes to.
  
          The setter-method creates a unique first transition to the state with the given id.
          Existing first transitions are removed. If the given state id is None, the first transition is removed.
  
          :return: The id of the start state
          """
          for transition_id in self.transitions:
              if self.transitions[transition_id].from_state is None:
                  to_state = self.transitions[transition_id].to_state
                  if to_state is not None:
                      return to_state
                  else:
                      return self.state_id
          return None