scheme :: FRAENKEL:sch 13
Gen299{ F₁() -> set , F₂() -> set , F₃() -> set , F₄( object , object ) -> Element of F₃(), P₁[ object , object ], P₂[ object ] } :

{ st1 where st1 is Element of F₃() : ( st1 in { F₄(s1,t1) where s1 is Element of F₁(), t1 is Element of F₂() : P₁[s1,t1] } & P₂[st1] ) } = { F₄(s2,t2) where s2 is Element of F₁(), t2 is Element of F₂() : ( P₁[s2,t2] & P₂[F₄(s2,t2)] ) }