ex Omega1, Omega2, Omega3, Omega4 being non empty set st
( Omega1 c< Omega2 & Omega2 c< Omega3 & Omega3 c< Omega4 & ex F1 being SigmaField of Omega1 ex F2 being SigmaField of Omega2 ex F3 being SigmaField of Omega3 ex F4 being SigmaField of Omega4 st
( F1 c= F2 & F2 c= F3 & F3 c= F4 ) )