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hi

 f(x,y)=y^2-x/2
 a=[]
 a.append(implicit_plot(f(x,y)==1,(x,-9,9),(y,-9,9)))
 a.append(implicit_plot(f(x,y)==2,(x,-9,9),(y,-9,9)))
 show(a[0]+a[1])

if i make a for, in order to plot several {f(x,y)=k}, how can i write the 'show' ?