<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def serialize(input, tree="etree", encoding=None, **serializer_opts):
 """Serializes the input token stream using the specified treewalker :arg input: the token stream to serialize :arg tree: the treewalker to use :arg encoding: the encoding to use :arg serializer_opts: any options to pass to the :py:class:`html5lib.serializer.HTMLSerializer` that gets created :returns: the tree serialized as a string Example: '<html><head></head><body><p>Hi!</p></body></html>' """

    # XXX: Should we cache this?
    walker = treewalkers.getTreeWalker(tree)
    s = HTMLSerializer(**serializer_opts)
    return s.render(walker(input), encoding)