scheme :: ALTCAT_1:sch 4
MSSLambda3D{ F1() -> non empty set , F2() -> non empty set , F3() -> non empty set , F4( object , object , object ) -> object } :
ex M being ManySortedSet of [:F1(),F2(),F3():] st
for i being Element of F1()
for j being Element of F2()
for k being Element of F3() holds M . (i,j,k) = F4(i,j,k)