Before revision:  An attempt to prove this result was presented by Fleischner, Mujuni, Paulusma, and Szeider Theor. Comput. Sci. 2009], but we realized that there was an apparently non-fixable flaw in their proof  .
After revision:  An attempt to prove this result was presented by Fleischner, Mujuni, Paulusma, and Szeider , 2009], but we realized that there was an apparently non-fixable flaw in their proof .