A Plan_3 is degenerate iff a = b = c *and* c = 0.

Kernel_23/doc_tex/Kernel_23_ref/Plane_3.tex

@@ -35,7 +35,8 @@ positive side of \ccStyle{h}, iff \ccTexHtml{$a\, px +b\, py +c\, pz +
                        const Kernel::RT &d)}
 {creates a plane \ccVar\ defined by the equation
  \ccTexHtml{$a\, px +b\, py +c\, pz + d = 0$}{a px + b py + c pz + d = 0}.
-Notice that \ccVar\ is degenerate if \ccTexHtml{$a = b = c$}{a = b = c}.}
+Notice that \ccVar\ is degenerate if 
+ \ccTexHtml{$a = b = c = 0$}{a = b = c = 0}.}
 
 \ccConstructor{Plane_3(const Point_3<Kernel> &p,
                        const Point_3<Kernel> &q,