for X1, X2, Y1, Y2 being set
for f1 being Function of X1,Y1
for f2 being Function of X2,Y2 st ( Y1 = {} implies X1 = {} ) & ( Y2 = {} implies X2 = {} ) holds
[:f1,f2:] = <:(f1 * (pr1 (X1,X2))),(f2 * (pr2 (X1,X2))):>