let Y0, Y1 be TopStruct ; :: thesis: for D0 being Subset of Y0
for D1 being Subset of Y1 st TopStruct(# the carrier of Y0, the topology of Y0 #) = TopStruct(# the carrier of Y1, the topology of Y1 #) & D0 = D1 & D0 is maximal_discrete holds
D1 is maximal_discrete
let D0 be Subset of Y0; :: thesis: for D1 being Subset of Y1 st TopStruct(# the carrier of Y0, the topology of Y0 #) = TopStruct(# the carrier of Y1, the topology of Y1 #) & D0 = D1 & D0 is maximal_discrete holds
D1 is maximal_discrete
let D1 be Subset of Y1; :: thesis: ( TopStruct(# the carrier of Y0, the topology of Y0 #) = TopStruct(# the carrier of Y1, the topology of Y1 #) & D0 = D1 & D0 is maximal_discrete implies D1 is maximal_discrete )
assume A1: TopStruct(# the carrier of Y0, the topology of Y0 #) = TopStruct(# the carrier of Y1, the topology of Y1 #) ; :: thesis: ( not D0 = D1 or not D0 is maximal_discrete or D1 is maximal_discrete )
assume A2: D0 = D1 ; :: thesis: ( not D0 is maximal_discrete or D1 is maximal_discrete )
assume A3: D0 is maximal_discrete ; :: thesis: D1 is maximal_discrete

A4: now :: thesis: for D being Subset of Y1 st D is discrete & D1 c= D holds
D1 = D

let D be Subset of Y1; :: thesis: ( D is discrete & D1 c= D implies D1 = D )
reconsider E = D as Subset of Y0 by A1;
assume D is discrete ; :: thesis: ( D1 c= D implies D1 = D )
then A5: E is discrete by A1, Th19;
assume D1 c= D ; :: thesis: D1 = D
hence D1 = D by A2, A3, A5; :: thesis: verum

end;

D0 is discrete by A3;
then D1 is discrete by A1, A2, Th19;
hence D1 is maximal_discrete by A4; :: thesis: verum