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"

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)