怎样使精灵只能横向拖动不能纵向拖动？

property pMoveable,pSprite

on mouseDown me
pMoveable=True
end

on mouseUp me
pMoveable=False
end

on mouseUpOutside me
pMoveable=False
end

on prepareFrame me
pSprite=sprite(me.spriteNum)
if pMoveable then
moveSprite
end if
end

on moveSprite
pSprite.locH=the mouseH
end

OK?