theorem Th49:
  the carrier of latt F = F & the L_join of latt F = (the L_join
  of L)||F & the L_meet of latt F = (the L_meet of L)||F
proof
  ex o1,o2 st o1 = (the L_join of L)||F & o2 = (the L_meet of L)||F & latt
  F = LattStr (#F, o1, o2#) by Def9;
  hence thesis;
end;
