reserve X,Y,Z,X1,X2,Y1,Y2 for set, x,y,z,t,x1,x2 for object,
  f,g,h,f1,f2,g1,g2 for Function;

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;
