Conditionally add column headers

[wonder95]

I have a use case where I am using both map() and headings() in my export, and I want to conditionally add columns depending on a condition within the row record. It's simple enough to add them in map() as needed, but it appears that I can't add the headings in headings(), because it's a read only method, and the row object isn't available. My thought was to add a property to the class, but that won't work, because the export is called on a per-export basis, not per record.

    public function export($start, $end)
    {
        return Excel::download(new TreasuryReportExport($start, $end), 'TreasuryReport.csv');
    }

Is there a way I can conditionally add my header rows on a per-record basis?