theorem Th7:
  the carrier of L is ClosedSubset of L
proof
  the carrier of L c= the carrier of L;
  then reconsider F=the carrier of L as Subset of L;
A1: p in F & q in F implies p "/\" q in F;
  p in F & q in F implies p "\/" q in F;
  hence thesis by A1,LATTICES:def 24,def 25;
end;
