<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def to_file(self, slug, folderpath=None, header=None, footer=None):
 """ Writes the html report to a file from the report stack """

        if folderpath is None:
            if self.report_path is None:
                self.err(
                    "Please set the report_path parameter or pass a path in arguments")
                return
            folderpath = self.report_path
        else:
            self.report_path = folderpath
        html = self._get_header(header)
        if html is None or html == "":
            self.err(self.to_file, "Can not get html header")
        for report in self.reports:
            if "html" not in report:
                self.err("No html for report " + report)
                self.reports = self.report_engines = []
                return
            html += report["html"]
        html += self._get_footer(footer)
        try:
            path = self._write_file(slug, folderpath, html)
            path = "file://" + path
        except Exception as e:
            self.err(e, self.to_file, "Can not save report to file")
            return
        self.reports = []
        self.report_engines = []
        if self.notebook is True:
            link = '<a href="' + path + '" target="_blank">' + path + '</a>'
            return display(HTML(link))