scheme :: FRAENKEL:sch 14
Gen39{ F₁() -> set , F₂( object ) -> object , P₁[ object ], P₂[ object ] } :

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