for f, g being Function of [:F₁(),F₂():],F₃() st ( for x, y being object st x in F₁() & y in F₂() holds
f . (x,y) = F₄(x,y) ) & ( for x, y being object st x in F₁() & y in F₂() holds
g . (x,y) = F₄(x,y) ) holds
f = g