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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 preprocess(content, options):
 # type: (Text, Optional[optparse.Values]) -> ReqFileLines """Split, filter, and join lines, and return a line iterator :param content: the content of the requirements file :param options: cli options """

    lines_enum = enumerate(content.splitlines(), start=1)  # type: ReqFileLines
    lines_enum = join_lines(lines_enum)
    lines_enum = ignore_comments(lines_enum)
    lines_enum = skip_regex(lines_enum, options)
    lines_enum = expand_env_variables(lines_enum)
    return lines_enum