scheme :: ALTCAT_1:sch 1
MSSLambda2{ F₁() -> set , F₂() -> set , F₃( object , object ) -> object } :

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