def add(self, elem, replace=False):
        """Add a requirement.

        :param elem: a string or :class:`Requirement` instance.
        :param replace: if `True`, packages in the set with the same name will
            be removed first.
        """
        if isinstance(elem, text_type):
            elem = Requirement.coerce(elem)
        if replace and elem in self:
            self.remove(elem)
        super(Requirements, self).add(elem)
def enable_plugin(plugin):
    plugin = get_plugin_from_all(plugin)

    if plugin.enabled:
        flash(_("Plugin %(plugin)s is already enabled.", plugin=plugin.name),
              "info")
        return redirect(url_for("management.plugins"))

    try:
        plugin.enable()
        flash(_("Plugin %(plugin)s enabled. Please restart FlaskBB now.",
                plugin=plugin.name), "success")
    except OSError:
        flash(_("It seems that FlaskBB does not have enough filesystem "
                "permissions. Try removing the 'DISABLED' file by "
                "yourself instead."), "danger")

    return redirect(url_for("management.plugins"))