User blog comment:Alemagno12/How strong is this function? (2)/@comment-25337554-20171111093003/@comment-1605058-20171111100150

I think something slightly stronger is meant: if (a,b) is in S, then both a, b are in S and both are smaller than (a,b). In symbols, ∀a,b: [ (a,b)∈S ⇒ a∈S ∧ b∈S ∧ a<(a,b) ∧ b<(a,b) ]