def psit(t, x, xf):
    """ A function of t, X_t and X_{t+1} (f=future) """
    if t == 0:
        return psi0(x) + psit(1, x, xf)
    else:
        return -0.5 / sigma0**2 + (0.5 / sigma0**4) * ((xf - mu0) - 
                                                       phi0 * (x - mu0))**2
def psit ( t , x , xf ) : if t == 0 : return psi0 ( x ) + psit ( 1 , x , xf ) else : return - 0.5 / sigma0 ** 2 + ( 0.5 / sigma0 ** 4 ) * ( ( xf - mu0 ) - phi0 * ( x - mu0 ) ) ** 2
A function of t, X_t and X_{t+1} (f=future)