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)
}