class AltParser[I, T](p: => Parser[I, T], 

                      q: => Parser[I, T]) 

                          extends Parser[I, T] {

  def parse(sb: I) = p.parse(sb) ++ q.parse(sb)   

}

class SeqParser[I, T, S](p: => Parser[I, T], 

                         q: => Parser[I, S]) 

                             extends Parser[I, (T, S)] {

  def parse(sb: I) = 

    for ((head1, tail1) <- p.parse(sb); 

         (head2, tail2) <- q.parse(tail1)) 

            yield ((head1, head2), tail2)

}

class FunParser[I, T, S](p: => Parser[I, T], f: T => S) 

                                  extends Parser[I, S] {

  def parse(sb: I) = 

    for ((head, tail) <- p.parse(sb)) 

      yield (f(head), tail)

}