:: deftheorem Def6 defines Closed_Domains_Union TDLAT_1:def 6 :
for T being TopSpace
for b₂ being BinOp of (Closed_Domains_of T) holds
( b₂ = Closed_Domains_Union T iff for A, B being Element of Closed_Domains_of T holds b₂ . (A,B) = A \/ B );