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

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