Definition 1.4.2.  Let S, T be sets, ≺ be a relation from S to T, s ∈ S, t ∈ T. We define
s ≺ t
by:
s [pu,v]u ∈ S,v ∈ T t  :⇔  ps,t  (pu,v is a proposition for each u ∈ S and v ∈ T)