def preprocess(content, options):<EOL>
lines_enum = enumerate(content.splitlines(), start=<NUM_LIT:1>)  <EOL>lines_enum = join_lines(lines_enum)<EOL>lines_enum = ignore_comments(lines_enum)<EOL>lines_enum = skip_regex(lines_enum, options)<EOL>lines_enum = expand_env_variables(lines_enum)<EOL>return lines_enum<EOL>
Split, filter, and join lines, and return a line iterator

    :param content: the content of the requirements file
    :param options: cli options