def make_little_d(x, order, mode):
    """
    Make the LP equation d' vector (Da = d')
    """
    if mode == 'f':
        return x[order:].reshape(len(x) - order, 1)
    elif mode == 'b':
        L = len(x) - order
        return x[:L].reshape(L, 1)
    else:
        raise ValueError("mode must be 'f' or 'b'")
