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

theorem
  a\(b\c)= (a\b)"\/"(a"/\"c)
proof
  thus a\(b\c) = a"/\"('not' b"\/"'not' ('not' c)) by Th36
    .= a"/\"('not' b"\/"c) by WAYBEL_1:87
    .= (a\b)"\/"(a"/\"c) by WAYBEL_1:def 3;
end;
