Question: Implicitplot problem

I want to plot  the astroid by the following code, but only the part in the first quadrant appears. I don't know that is wrong and how to solve the problem. Thanks.

with(plots):
a:=1:
implicitplot(x^(2/3)+y^(2/3)=a^(2/3),x=-a..a,y=-a..a);


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