scheme :: YELLOW_3:sch 6
Kappa2DS{ F1() -> non empty RelStr , F2() -> non empty RelStr , F3() -> non empty RelStr , F4( set , set ) -> set } :
ex f being Function of [:F1(),F2():],F3() st
for x being Element of F1()
for y being Element of F2() holds f . (x,y) = F4(x,y)
provided
A1: for x being Element of F1()
for y being Element of F2() holds F4(x,y) is Element of F3()