 reserve L for Quasi-Boolean_Algebra,
         x, y, z for Element of L;

theorem Th1:
  (x "\/" y)` = x` "/\" y`
  proof
    (x` "/\" y`)` = x`` "\/" y`` by Def1;
    hence x` "/\" y` = (x`` "\/" y``)` by ROBBINS3:def 6
              .= (x "\/" y``)` by ROBBINS3:def 6
              .= (x "\/" y)` by ROBBINS3:def 6;
  end;
