 reserve L for Quasi-Boolean_Algebra,
         x, y, z for Element of L;
 reserve L for Nelson_Algebra,
         a, b, c, d, x, y, z for Element of L;

theorem Th37: :: (2.39)
  -(a "\/" b) = ((-a) "/\" (-b))
  proof
    (a "\/" b)` = (a`` "\/" b)` by ROBBINS3:def 6
               .= (a`` "\/" b``)` by ROBBINS3:def 6
               .= (a` "/\" b`)`` by Def1
               .= (a` "/\" b`) by ROBBINS3:def 6;
    hence thesis;
 end;
