reserve L for Boolean non empty RelStr;
reserve a,b,c,d for Element of L;

theorem
  a"\/"b = (a\b)"\/"b
proof
  thus (a\b)"\/"b = (b"\/"a)"/\"(b"\/"'not' b) by Th17
    .= (b"\/"a)"/\" Top L by Th34
    .= a"\/"b by WAYBEL_1:4;
end;
