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

theorem
  (a"/\"b)"\/"(a\b) = a
proof
  thus (a"/\"b)"\/"(a\b) = ((a"/\"b)"\/"a)"/\"((a"/\"b)"\/"'not' b) by Th17
    .= a"/\"((a"/\"b)"\/"'not' b) by LATTICE3:17
    .= a"/\"(('not' b"\/"a)"/\"('not' b"\/"b)) by Th17
    .= a"/\"(('not' b"\/"a)"/\"Top L) by Th34
    .= a"/\"('not' b"\/"a) by WAYBEL_1:4
    .= a by LATTICE3:18;
end;
