scheme :: YELLOW_9:sch 4
ABC{ F₁() -> TopSpace, F₂( set ) -> set , F₃() -> Function, P₁[ set ] } :

F₃() " (union { F₂(x) where x is Subset of F₁() : P₁[x] } ) = union { (F₃() " F₂(y)) where y is Subset of F₁() : P₁[y] }