Updates the indices of all buses.

        @param start: Starting index, typically 0 or 1.
        @type start: int

def index_buses(self, buses=None, start=0):
        """ Updates the indices of all buses.

        @param start: Starting index, typically 0 or 1.
        @type start: int
        """
        bs = self.connected_buses if buses is None else buses
        for i, b in enumerate(bs):
            b._i = start + i