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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 from_file(cls, f, filename=None, includedir='', seenfiles=None):
        '''Create a token stream by reading an input file

        Read tokens from `f`. If an include directive ('@include "file.cfg"')
        is found, read its contents as well.

        The `filename` argument is used for error messages and to detect
        circular imports. ``includedir`` sets the lookup directory for included
        files.  ``seenfiles`` is used internally to detect circular includes,
        and should normally not be supplied by users of is function.
        '''

        if filename is None:
            filename = getattr(f, 'name', '<unknown>')
        if seenfiles is None:
            seenfiles = set()

        if filename in seenfiles:
            raise ConfigParseError("Circular include: %r" % (filename,))
        seenfiles = seenfiles | {filename}  # Copy seenfiles, don't alter it.

        tokenizer = Tokenizer(filename=filename)
        lines = []
        tokens = []
        for line in f:
            m = re.match(r'@include "(.*)"$', line.strip())
            if m:
                tokens.extend(tokenizer.tokenize(''.join(lines)))
                lines = [re.sub(r'\S', ' ', line)]

                includefilename = decode_escapes(m.group(1))
                includefilename = os.path.join(includedir, includefilename)
                try:
                    includefile = open(includefilename, "r")
                except IOError:
                    raise ConfigParseError("Could not open include file %r" %
                                           (includefilename,))

                with includefile:
                    includestream = cls.from_file(includefile,
                                                  filename=includefilename,
                                                  includedir=includedir,
                                                  seenfiles=seenfiles)
                tokens.extend(includestream.tokens)

            else:
                lines.append(line)

        tokens.extend(tokenizer.tokenize(''.join(lines)))
        return cls(tokens)