def METHOD_NAME(self, group=None):
    """Creates the linear constraint for the class:`ElectricalLine`
    block.
    Parameters
    ----------
    group : list
        List of oemof.solph.ElectricalLine (eline) objects for which
        the linear relation of inputs and outputs is created
        e.g. group = [eline1, eline2, ...]. The components inside the
        list need to hold a attribute `reactance` of type Sequence
        containing the reactance of the line.
    """
    if group is None:
        return None
    m = self.parent_block()
    # create voltage angle variables
    self.ELECTRICAL_BUSES = Set(
        initialize=[n for n in m.es.nodes if isinstance(n, ElectricalBus)]
    )
    def _voltage_angle_bounds(block, b, t):
        return b.v_min, b.v_max
    self.voltage_angle = Var(
        self.ELECTRICAL_BUSES, m.TIMESTEPS, bounds=_voltage_angle_bounds
    )
    if True not in [b.slack for b in self.ELECTRICAL_BUSES]:
        # TODO: Make this robust to select the same slack bus for
        # the same problems
        bus = [b for b in self.ELECTRICAL_BUSES][0]
        logging.info(
            "No slack bus set,setting bus {0} as slack bus".format(
                bus.label
            )
        )
        bus.slack = True
    def _voltage_angle_relation(block):
        for p, t in m.TIMEINDEX:
            for n in group:
                if n.input.slack is True:
                    self.voltage_angle[n.output, t].value = 0
                    self.voltage_angle[n.output, t].fix()
                try:
                    lhs = m.flow[n.input, n.output, p, t]
                    rhs = (
                        1
                        / n.reactance[t]
                        * (
                            self.voltage_angle[n.input, t]
                            - self.voltage_angle[n.output, t]
                        )
                    )
                except ValueError:
                    raise ValueError(
                        "Error in constraint creation",
                        "of node {}".format(n.label),
                    )
                block.electrical_flow.add((n, p, t), (lhs == rhs))
    self.electrical_flow = Constraint(group, m.TIMEINDEX, noruleinit=True)
    self.electrical_flow_build = BuildAction(rule=_voltage_angle_relation)