public List <Footballer> ListAll()
 {
     return(List_footballers.OrderBy(a => a.Name).ToList());
 }
 public List <Footballer> ListOverThirty()
 {
     return(List_footballers.Where(f => f.Age > 30)
            .OrderByDescending(f => f.Level).ToList());
 }
 public void AddFootballer(Footballer f)
 {
     List_footballers.Add(f);
     Team_level += f.Level * Team_coach.Level;
 }