docs: def _build_graph(self) -> nx.DiGraph:
          """ Private method to build the graph from the model. """
          digraph = nx.DiGraph()
          for state in self.model.all_states():
              self._number_of_states += 1
              for next_state in self.model.available_state(state):
                  self._number_of_transitions += 1
                  digraph.add_edge(
                      self._transform_state_to_string(state),
                      self._transform_state_to_string(next_state)
                  )
          return digraph