Question: plot option adaptive

HI, dear all. When I tried to use the plot option 'adaptive' to make my plot more smooth and realistic, I encountered the following erro. I cannot understand why. From the help guide, I learn that adaptive can be assigned n or true or false, but errors appeared.  Thanks for your help.

 

> implicitplot(-x^3+3*x+a = 0, a = -3 .. 3, x = -4.0 .. 4.0, view = [-3 .. 3, -4 .. 4], adaptive = 2, resolution = 1000, numpoints = 2000);
Error, (in plot/options2d) unexpected option: adaptive = 2
> help("adaptive");

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