def goto_file_line(self, event=None):
        """Handle request to open file/line.

        If the selected or previous line in the output window
        contains a file name and line number, then open that file
        name in a new window and position on the line number.

        Otherwise, display an error messagebox.
        """
        line = self.text.get("insert linestart", "insert lineend")
        result = file_line_helper(line)
        if not result:
            # Try the previous line.  This is handy e.g. in tracebacks,
            # where you tend to right-click on the displayed source line
            line = self.text.get("insert -1line linestart",
                                 "insert -1line lineend")
            result = file_line_helper(line)
            if not result:
                self.showerror(
                    "No special line",
                    "The line you point at doesn't look like "
                    "a valid file name followed by a line number.",
                    parent=self.text)
                return
        filename, lineno = result
        self.flist.gotofileline(filename, lineno)