set D = [:(Open_Domains_of T),(Open_Domains_of T):];
defpred S₁[ set , set ] means for A, B being Element of Open_Domains_of T st $1 = [A,B] holds
$2 = Int (Cl (A \/ B));
A1: for a being Element of [:(Open_Domains_of T),(Open_Domains_of T):] ex b being Element of Open_Domains_of T st S₁[a,b]

proof

let a be Element of [:(Open_Domains_of T),(Open_Domains_of T):]; :: thesis: ex b being Element of Open_Domains_of T st S₁[a,b]
reconsider G = a `1 , F = a `2 as Element of Open_Domains_of T ;
Int (Cl (G \/ F)) is open_condensed by Th23;
then Int (Cl (G \/ F)) in { E where E is Subset of T : E is open_condensed } ;
then reconsider b = Int (Cl (G \/ F)) as Element of Open_Domains_of T ;
take b ; :: thesis: S₁[a,b]
let A, B be Element of Open_Domains_of T; :: thesis: ( a = [A,B] implies b = Int (Cl (A \/ B)) )
assume a = [A,B] ; :: thesis: b = Int (Cl (A \/ B))
then [A,B] = [G,F] by MCART_1:23;
then ( A = G & B = F ) by ZFMISC_1:33;
hence b = Int (Cl (A \/ B)) ; :: thesis: verum

end;

consider h being Function of [:(Open_Domains_of T),(Open_Domains_of T):],(Open_Domains_of T) such that
A2: for a being Element of [:(Open_Domains_of T),(Open_Domains_of T):] holds S₁[a,h . a] from FUNCT_2:sch 3(A1);
take h ; :: thesis: for A, B being Element of Open_Domains_of T holds h . A,B = Int (Cl (A \/ B))
let A, B be Element of Open_Domains_of T; :: thesis: h . A,B = Int (Cl (A \/ B))
thus h . A,B = h . [A,B]
.= Int (Cl (A \/ B)) by A2 ; :: thesis: verum