scheme :: FRAENKEL:sch 15
Gen399{ F₁() -> set , F₂() -> set , F₃( object , object ) -> object , P₁[ object , object ], P₂[ object ] } :

{ F₃(s,t) where s is Element of F₁(), t is Element of F₂() : ( s in { s1 where s1 is Element of F₁() : P₂[s1] } & P₁[s,t] ) } = { F₃(s2,t2) where s2 is Element of F₁(), t2 is Element of F₂() : ( P₂[s2] & P₁[s2,t2] ) }