[Maxima] plot3d multiple functions bug?

[Krzysztof Leśniak]

I cannot obtain the 3d plotting of  2 functions (and  more functions too):
 
plot3d([7*x-3*y, x^2-y^2], [x,-5,5], [y,-5,5])$

List [7*x-3*y,x^2-y^2] is not of length 3 -- an error.  To debug this 
try debugmode(true);

Is this a bug?
My student needed urgently this feature. Sorry if it is known issue.
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Krzysztof Le`sniak