reserve i,j,k,n for Nat;
reserve x,x1,x2,x3,y1,y2,y3 for set;

theorem Th4:
  for X being set holds card id X = card X
proof
  let X be set;
  id X in Funcs(X,X) by FUNCT_2:9;
  hence thesis by CARD_2:3;
end;
