On 21 Mrz., 11:36, YBM <ybm...@nooos.fr.invalid> wrote: > Le 21.03.2013 07:54, WM a crit : > > > Every not necessary line can be removed. Why do you think there should > > remain unnecessary lines? And if they are needed, which it the first > > unnecessary line that must remain? > > Which is the leg which is necessary in order for a dog to have more > than two legs? > Try to understand set inclusion.