for X being set
for S being SigmaField of X
for G, F being sequence of S st G . 0 = {} & ( for n being Nat holds
( G . (n + 1) = (F . 0) \ (F . n) & F . (n + 1) c= F . n ) ) holds
union (rng G) = (F . 0) \ (meet (rng F))