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()