scheme :: PBOOLE:sch 1
KuratowskiFunction{ F₁() -> set , F₂( object ) -> set } :

ex f being ManySortedSet of F₁() st
for e being object st e in F₁() holds
f . e in F₂(e)

provided

A1: for e being object st e in F₁() holds
F₂(e) <> {}