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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 _group(tlist, cls, match, valid_prev=lambda t: True, valid_next=lambda t: True, post=None, extend=True, recurse=True ):
 """Groups together tokens that are joined by a middle token. i.e. x < y"""


    tidx_offset = 0
    pidx, prev_ = None, None
    for idx, token in enumerate(list(tlist)):
        tidx = idx - tidx_offset

        if token.is_whitespace:
            continue

        if recurse and token.is_group and not isinstance(token, cls):
            _group(token, cls, match, valid_prev, valid_next, post, extend)

        if match(token):
            nidx, next_ = tlist.token_next(tidx)
            if prev_ and valid_prev(prev_) and valid_next(next_):
                from_idx, to_idx = post(tlist, pidx, tidx, nidx)
                grp = tlist.group_tokens(cls, from_idx, to_idx, extend=extend)

                tidx_offset += to_idx - from_idx
                pidx, prev_ = from_idx, grp
                continue

        pidx, prev_ = tidx, token