scheme :: KNASTER:sch 1
Knaster{ F₁() -> set , F₂( object ) -> set } :

ex D being set st
( F₂(D) = D & D c= F₁() )

provided

A1: for X, Y being set st X c= Y holds
F₂(X) c= F₂(Y) and
A2: F₂(F₁()) c= F₁()