Exponential weighted moving variance.

def var(self, bias=False, *args, **kwargs):
        """
        Exponential weighted moving variance.
        """
        nv.validate_window_func('var', args, kwargs)

        def f(arg):
            return libwindow.ewmcov(arg, arg, self.com, int(self.adjust),
                                    int(self.ignore_na), int(self.min_periods),
                                    int(bias))

        return self._apply(f, **kwargs)