Question: implicitplot3d real and imaginary part

How do I plot the imaginary or real part of a complex valued function

e.g.

E:=y^2+x^2=1

I tried:

E1:=eval(E,{y=y1+I*y2,x=x1+I*x2})

implicitplot3d(E1,x1=-3..3,x2=-3..3,y1=-3..3)

but which does not work

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