scheme :: TOPALG_7:sch 1
TopFuncEx{ F1() -> non empty TopSpace, F2() -> non empty TopSpace, F3() -> non empty set , F4( Point of F1(), Point of F2()) -> Element of F3() } :
ex f being Function of [:F1(),F2():],F3() st
for s being Point of F1()
for t being Point of F2() holds f . (s,t) = F4(s,t)