reserve T for TopSpace;
reserve A,B for Subset of T;
reserve T for non empty TopSpace;
reserve P,Q for Element of Topology_of T;
reserve p,q for Element of Open_setLatt(T);

theorem Th6:
  p [= q iff p c= q
proof
  p "\/" q = p \/ q by Def2;
  hence thesis by XBOOLE_1:7,12;
end;
