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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 _get_11paths_serialized_headers(x_headers):
 """ Prepares and returns a string ready to be signed from the 11-paths specific HTTP headers received. :param x_headers: a non necessarily ordered map (array without duplicates) of the HTTP headers to be ordered. :return: string The serialized headers, an empty string if no headers are passed, or None if there's a problem such as non 11paths specific headers """

    if x_headers:
        headers = to_key_val_list(x_headers, sort=True, insensitive=True)
        serialized_headers = ""
        for key, value in headers:
            if key.lower().startswith(X_11PATHS_HEADER_PREFIX.lower()) and \
                            key.lower() != X_11PATHS_DATE_HEADER_NAME.lower():
                serialized_headers += key.lower() + X_11PATHS_HEADER_SEPARATOR + value.replace("\n", " ") + " "
        return serialized_headers.strip()
    else:
        return ""