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

theorem Th9:
  a"\/"(b"/\"c) = (a"\/"b)"/\"(a"\/"c)
proof
  for a,b,c holds a"/\"(b"\/"c) = (a"/\"b)"\/"(a"/\"c) by Def11;
  hence thesis by Th1;
end;
