theorem Th78:
  D1 c= D2 & the TopStruct of X1 = the TopStruct of X2 implies Int D1 c= Int D2
proof
  assume
A1: D1 c= D2;
  then reconsider C2 = D1 as Subset of X2 by XBOOLE_1:1;
  assume the TopStruct of X1 = the TopStruct of X2;
  then Int D1 = Int C2 by Th77;
  hence thesis by A1,TOPS_1:19;
end;
