如何在SymPy中扩展矩阵表达式？

在SymPy中,我试图执行矩阵乘法并在之后展开它.但是,SymPy似乎不支持矩阵表达式的扩展.例如,这是矩阵的4阶Runge-Kutta(RK4):

from sympy import init_session
init_session()
from sympy import *

A = MatrixSymbol('A', 3, 3)
x = MatrixSymbol('x', 3, 1)
dt = symbols('dt')

k1 = A*x
k2 = A*(x + S(1)/2*k1*dt)
k3 = A*(x + S(1)/2*k2*dt)
k4 = A*(x + k3*dt)
final = dt*S(1)/6*(k1 + 2*k2 + 2*k3 + k4)
final.expand()

Traceback (most recent call last)
 in ()
     12 final = dt*1/6*(k1+2*k2+2*k3+k4)
     13 
---> 14 final.expand()

AttributeError: 'MatMul' object has no attribute 'expand'

A,x,dt = symbols('A x dt')
k1 = A*x
k2 = A*(x+k1*dt*S(1)/2)
k3 = A*(x+k2*dt*S(1)/2)
k4 = A*(x+k3*dt)
final = x+dt*(S(1)/6)*(k1+k2+k3+k4)
collect(expand((final)),x)

x*(A**4*dt**4/24 + A**3*dt**3/8 + A**2*dt**2/3 + 2*A*dt/3 + 1)