plot [-(x^2/2)-1/sqrt(x^2+y^2)=-1.5,-(x^2/2)-1/sqrt(x^2+y^2)=-1.55,-(x^2/2)-1/sqrt(x^2+y^2)=-1.6,-(x^2/2)-1/sqrt(x^2+y^2)=-1.65] for x=-1 to 1

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