class linkst:
    __slots__ = ['data', 'next']
    def __init__(self, iterable=(), data=None, next=None):
        if iterable is not None:
            self.data, self.next = self, None
            self.extend(iterable)
        else:
            self.data, self.next = data, next

    def empty(self):
        return self.next is None

    def append(self, data):
        last = self.data
        self.data = last.next = linkst(None, data)


    def insert(self, data, index=0):
        curr, cat = self, 0
        while cat < index and curr:
            curr, cat = curr.next, cat+1
        if index<0 or not curr:
            raise IndexError(index)
        new = linkst(None, data, curr.next)
        if curr.next is None: self.data = new
        curr.next = new

    def reverse(self):
        current, prev = self.next, None
        while current:
            next = current.next
            current.next = prev
            prev, current = current, next
        if self.next: self.data = self.next
        self.next = prev

    def delete(self, index=0):
        curr, cat = self, 0
        while cat < index and curr.next:
            curr, cat = curr.next, cat+1
        if index<0 or not curr.next:
            raise IndexError(index)
        curr.next = curr.next.next
        if curr.next is None:
            self.data = curr

    def remove(self, data):
        current = self
        while current.next:
            if data == current.next.data: break
            current = current.next
        else: raise ValueError(data)
        current.next = current.next.next
        if current.next is None:
            self.data = current

    def __contains__(self, data):
        current = self.next
        while current:
            if data == current.data:
                return True
            current = current.next
        return False

    def __iter__(self):
        itr = linkst()
        itr.next = self.next
        return itr

    def __next__(self):
        if self.data is not self or self.next is None:
            raise StopIteration()
        next = self.next
        self.next = next.next
        return next.data

    def __repr__(self):
        return 'linkst(%r)'%list(self)

    def __str__(self):
        return '->'.join(str(i) for i in self)


    def extend(self, iterable):
        last = self.data
        for i in iterable:
            last.next = linkst(None, i)
            last = last.next
        self.data = last

    def index(self, data):
        current, idx = self.next, 0
        while current:
            if current.data == data: return idx
            current, idx = current.next, idx+1
        raise ValueError(data)

a=linkst()
a.append(6)
a.append(5)
a.append(5)
a.append(12)
a.append(32)
print(a)
a.remove(5)
print(a)
for i in a:
    print (i)
if 6 in a:
    print("123")