abstract class Parser[I, T] {

  def parse(ts: I): Set[(T, I)]

  def parse_all(ts: I) : Set[T] =

    for ((head, tail) <- parse(ts); if (tail.isEmpty)) 

      yield head

  def || (right : => Parser[I, T]) : Parser[I, T] = 

      new AltParser(this, right)

  def ==>[S] (f: => T => S) : Parser [I, S] = 

      new FunParser(this, f)

   def ~[S] (right : => Parser[I, S]) : Parser[I, (T, S)] = 

      new SeqParser(this, right)

}