Validation of edge cases and incompatible options

def _validate(cls, options, items, warnfn):
        "Validation of edge cases and incompatible options"

        if 'html' in Store.display_formats:
            pass
        elif 'fig' in items and items['fig'] not in Store.display_formats:
            msg = ("Requesting output figure format %r " % items['fig']
                   + "not in display formats %r" % Store.display_formats)
            if warnfn is None:
                print('Warning: {msg}'.format(msg=msg))
            else:
                warnfn(msg)

        backend = Store.current_backend
        return Store.renderers[backend].validate(options)