reserve L for D_Lattice;
reserve a, b, c for Element of L;
reserve L for B_Lattice;
reserve a, b for Element of L;

theorem
  ( a"\/"b )` = a`"/\" b`
proof
  thus (a"\/"b)` = (a``"\/"b``)`
    .= (a`"/\"b`)`` by Th21
    .= a`"/\"b`;
end;
