scheme :: MULTOP_1:sch 3
Lambda3D{ F₁() -> non empty set , F₂() -> non empty set , F₃() -> non empty set , F₄() -> non empty set , F₅( Element of F₁(), Element of F₂(), Element of F₃()) -> Element of F₄() } :

ex f being Function of [:F₁(),F₂(),F₃():],F₄() st
for x being Element of F₁()
for y being Element of F₂()
for z being Element of F₃() holds f . [x,y,z] = F₅(x,y,z)