problem on setting posematrix

hi guys,
I am recently working on animating a robot with bones.
but there is something wierd when I tried to set poseMatrix of each bone.

import Blender
from Blender import *
print "
mat = Mathutils.RotationMatrix(30, 4, “z”)
ObjLLeg = Object.Get(“LLEG”)
ObjLLegPose = ObjLLeg.getPose()
LLegPoseBones = ObjLLegPose.bones
LULegPoseBone = LLegPoseBones[“LULEG”]
print LULegPoseBone.poseMatrix
mat = mat * LULegPoseBone.poseMatrix
for i in range(4):
for j in range(4):
LULegPoseBone.poseMatrix[i][j] = mat[i][j]

With the last sentence commented, I can redraw the bone, but the mesh.
With it uncommented, nothing will happen!