for Omega being non empty set
for Sigma being SigmaField of Omega
for T being Nat
for MyFunc being Filtration of StoppingSet T,Sigma
for k being Function of Omega,(StoppingSetExt T) holds
( k is_StoppingTime_wrt MyFunc,T iff for t being Element of StoppingSet T holds { w where w is Element of Omega : k . w <= t } in MyFunc . t )