def add_bond(self, atom1, atom2, bond):
        """
        implementation of bond addition
        """
        if atom1 == atom2:
            raise KeyError('atom loops impossible')
        if atom1 not in self._node or atom2 not in self._node:
            raise KeyError('atoms not found')
        if atom1 in self._adj[atom2]:
            raise KeyError('atoms already bonded')

        attr_dict = self.edge_attr_dict_factory()
        if isinstance(bond, int):
            attr_dict.order = bond
        else:
            attr_dict.update(bond)

        self._adj[atom1][atom2] = self._adj[atom2][atom1] = attr_dict
        self.flush_cache()
implementation of bond addition