private void EnsureContains(Animal arg1, Limb arg2)
 {
     if (!arg1.Limbs.Contains(arg2))
         arg1.Limbs.Add(arg2);
 }
 private void EnsureContainsNot(Animal arg1, Limb arg2)
 {
     if (arg1.Limbs.Contains(arg2))
         arg1.Limbs.Remove(arg2);
 }