[x,y,z]=meshgrid(linspace(-1,1));v=x.^2+y.^2+z.^2-1;[xx,zz]=meshgrid(linspace(-2,1));yy=1-xx;h=contourslice(x,y,z,v,xx,yy,zz,[0,0]);set(h,'edgecolor','b');view(3);axis equal;grid on