scheme :: ALTCAT_2:sch 2
DomOnSingletons{ F₁() -> non empty set , F₂() -> Function, F₃( set ) -> set , P₁[ set ] } :

dom F₂() = { o where o is Element of F₁() : P₁[o] }

provided

A1: F₂() = { [o,F₃(o)] where o is Element of F₁() : P₁[o] }