reserve a,b,c,k,l,m,n for Nat,
  i,j,x,y for Integer;

theorem Th11: ::: KNASTER:13
  for A be set, B being non empty set,
      f being Function of A, B st f is bijective holds card A = card B
proof
  let A be set, B be non empty set,
      f be Function of A, B;
  assume f is bijective; then
A1: f is one-to-one onto;
A2: A = dom f by FUNCT_2:def 1;
  B c= rng f by A1;
  then A3: card B c= card A by A2,CARD_1:12;
  card A c= card B by A2,A1,CARD_1:10;
  hence thesis by A3;
end;
