>>14
Oh, I though tD returned true when all three are not the same. After reading >>16, I don't think there is an actual solution to this unless you know beforehand that a,b,c,d are all not a specific value. If you know that, then you can do

frE(a,b,c,d) = not( thrD(a,b,k) or thrD(a,c,k) or ...[rest of the pairs])

Otherwise, I don't see any solution.