theorem
  for X being trivial set, Y being set st X,Y are_equipotent holds Y is trivial
  proof
    let X be trivial set, Y be set such that
A1: X,Y are_equipotent;
A2: card X < 2 by NAT_D:60;
A3: Y is finite by A1,CARD_1:38;
    card X = card Y by A1,CARD_1:5;
    hence thesis by A2,A3,NAT_D:60;
  end;
