So I'm trying to get around the fact that plotting half of an
ellipsoid with plot3d is nigh impossible,

x,y=var('x,y')
plot3d(sqrt(1-(x^2/9+y^2/4)),(x,-3,3),(y,-2,2))

but I stumbled across this problem in the process.  Since the problem
is that we can't evaluate at some points lets replace them.  So trying
this

sqrt(max(1-(x^2/9+y^2/4),0))

returns this

sqrt(-1/9*x^2 - 1/4*y^2 + 1)

So passing this into plot3d doesn't fix anything.

Is it possible to have max behave as you would expect with a symbolic
expression, i.e. wait until you evaluate it or restrict the domain  to
check what is the maximum of the two or more values.

Thanks,
Matt
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