scheme :: SUBSET_1:sch 6
Union2{ F₁() -> set , F₂() -> Element of F₁(), F₃() -> Element of F₁(), F₄( object ) -> set } :

union { F₄(j) where j is Element of F₁() : j in {F₂(),F₃()} } = F₄(F₂()) \/ F₄(F₃())