scheme :: ALTCAT_1:sch 3
MSSLambda3{ F₁() -> set , F₂() -> set , F₃() -> set , F₄( object , object , object ) -> object } :

ex M being ManySortedSet of [:F₁(),F₂(),F₃():] st
for i, j, k being set st i in F₁() & j in F₂() & k in F₃() holds
M . (i,j,k) = F₄(i,j,k)