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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def as_asciitree(self, str_func=None):
 """Represent this Sentence as an ASCII tree string. Requires the asciitree package. A default token stringifier is provided but for custom formatting, specify a str_func which should take a single Token and return a string."""

        import asciitree
        from collections import defaultdict
        children = defaultdict(list)
        # since erased nodes may be missing, multiple tokens may have same
        # index (CCprocessed), etc.
        token_to_index = {}
        roots = []
        for token in self:
            children[token.head].append(token)
            token_to_index[token] = token.index
            if token.head == 0:
                roots.append(token)
        assert roots, "Couldn't find root Token(s)"

        if len(roots) > 1:
            # multiple roots so we make a fake one to be their parent
            root = Token(0, 'ROOT', 'ROOT-LEMMA', 'ROOT-CPOS', 'ROOT-POS',
                         None, None, 'ROOT-DEPREL', None, None, None)
            token_to_index[root] = 0
            children[0] = roots
        else:
            root = roots[0]

        def child_func(token):
            index = token_to_index[token]
            return children[index]
        if not str_func:
            def str_func(token):
                return ' %s [%s]' % (token.form, token.deprel)

        return asciitree.draw_tree(root, child_func, str_func)