docs: def _transform_state_to_string(self, state: State) -> str:
          """
          Private method which transform a state to a string.
  
          Examples
          --------
  
          The model contains 2 genes: operon = {0, 1, 2}
                                      mucuB = {0, 1}
  
          >>> graph._transform_state_to_string({operon: 1, mucuB: 0})
          "10"
          >>> graph._transform_state_to_string({operon: 2, mucuB: 1})
          "21"
  
          """
          return ''.join(str(state[gene]) for gene in self.model.genes)