Updates the indices of all branches.

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

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

        @param start: Starting index, typically 0 or 1.
        @type start: int
        """
        ln = self.online_branches if branches is None else branches
        for i, l in enumerate(ln):
            l._i = start + i