theorem
  card X1 = card Y1 & card X2 = card Y2 implies card Funcs(X1,X2) = card
  Funcs(Y1,Y2)
proof
  assume card X1 = card Y1 & card X2 = card Y2;
  then X1,Y1 are_equipotent & X2,Y2 are_equipotent by CARD_1:5;
  hence thesis by Th53;
end;
