def _check_signal(self, s):
        r"""Check if signal is valid."""
        s = np.asanyarray(s)
        if s.shape[0] != self.n_vertices:
            raise ValueError('First dimension must be the number of vertices '
                             'G.N = {}, got {}.'.format(self.N, s.shape))
        return s
r"""Check if signal is valid.