:: deftheorem Def4 defines thin PROB_4:def 4 :
for Omega being non empty set
for Sigma being SigmaField of Omega
for P being Probability of Sigma
for b₄ being Subset of Omega holds
( b₄ is thin of P iff ex A being set st
( A in Sigma & b₄ c= A & P . A = 0 ) );